Every VA disability claim that gets granted — every single one — runs through one of four doors. The VA calls them theories of service connection. Most veterans only know the first door (the condition started in service), and they lose claims they would have won if they had walked through one of the other three. Understanding all four is the single most useful thing you can learn about how the system works.
This is the foundational article. Once you know which path your condition fits — and that you can argue more than one at a time — the rest of the process (evidence, exams, ratings) clicks into place. Here are the four ways to get a condition service-connected, what each one requires, and how to choose.
The four paths at a glance
- 1. Direct — it started in service (38 CFR § 3.303).
- 2. Secondary — it was caused or worsened by an already service-connected condition (§ 3.310).
- 3. Presumptive — the law presumes the link for your condition + service (§ 3.307 / § 3.309 / § 3.320).
- 4. Aggravation — a pre-existing condition was permanently worsened by service (§ 3.306).
Path 1 — Direct service connection (§ 3.303)
This is the default path and the one everyone pictures: something happened in service, and you have a condition because of it. Under 38 CFR § 3.303, direct service connection has three elements:
- A current disability — a present, diagnosed condition. No current diagnosis, no claim.
- An in-service event, injury, or disease — documented in service treatment records, your DD-214, deployment records, or established through a lay/buddy statement.
- A medical nexus — an opinion that the current disability is “at least as likely as not” (50% or greater probability) related to the in-service event.
The nexus is where direct claims live or die. For conditions a layperson can observe over time (ringing in the ears, a trick knee that has bothered you since a fall), continuity of symptoms can substitute for a formal nexus opinion — a documented, unbroken line of the same symptom from service to now. For a deeper look at how the C&P examiner evaluates the nexus and what to say, see the C&P exam guide.
Path 2 — Secondary service connection (§ 3.310)
This is the most under-used path, and often the most lucrative. Under 38 CFR § 3.310, a new condition can be service-connected if it was caused by — or, under § 3.310(b), aggravated by — a condition that is already service-connected. You do not link it to service; you link it to your existing rated disability.
The classic, winnable examples:
- Sleep apnea secondary to PTSD — a well-documented physiologic pathway; see the full breakdown in the sleep apnea secondary to PTSD article.
- Depression or anxiety secondary to chronic pain — mental-health conditions flowing from a service-connected orthopedic disability.
- Radiculopathy secondary to a service-connected back — nerve involvement radiating from the spine.
Secondary claims need a current diagnosis of the secondary condition and a nexus opinion connecting it to the primary. Because so many conditions cascade from one another, mapping your secondaries is one of the highest-yield things you can do. The secondary conditions hub and the Secondary Conditions Mapper walk the common linkages and the evidence each one needs.
Path 3 — Presumptive service connection (§ 3.307 / 3.309 / 3.320)
Sometimes the law does the hardest work for you. For certain combinations of condition and service, the VA presumes service connection — the nexus is built into the regulation and you never have to prove it. The major presumptive categories:
- Chronic diseases within one year of separation (§ 3.309(a)) — e.g., arthritis, hypertension, certain neurological conditions manifesting to a compensable degree within a year.
- Agent Orange / herbicide exposure conditions for Vietnam-era and other covered veterans.
- Gulf War undiagnosed illnesses and chronic multisymptom illnesses.
- PACT Act burn-pit and airborne-hazard conditions under § 3.320 — covered in full in the PACT Act burn pit guide.
- Radiation-exposed veterans and former POWs for specified conditions.
Always check for a presumption first
If a presumption applies, lead with it. It removes the nexus — the most expensive and uncertain element of any claim. Browse what may already be presumed for your service on the presumptive conditions hub before you pay for a private nexus opinion you may not need.
Path 4 — Aggravation (§ 3.306)
The fourth path is the least understood. Under 38 CFR § 3.306, if you entered service with a pre-existing condition that was noted at entry, and your service permanently worsened it beyond its natural progression, the increase in disability can be service-connected. Key points:
- There is a presumption of aggravation when a pre-existing condition worsens during service.
- The VA can only rebut it with clear and unmistakable evidence that the worsening was due to the natural progression of the condition, not service.
- A condition not noted at entry is generally presumed to have been sound at entry (the presumption of soundness) — which can push the claim back toward a direct theory instead.
Aggravation matters for veterans told “that was a pre-existing condition” and denied. If service made it worse, the worsening is potentially compensable.
How to choose your path (and why you can plead more than one)
These paths are not mutually exclusive. A well-built claim often argues alternative theories so the VA is required to consider each. A practical decision order:
- Check for a presumption first. If one applies, it is the cheapest route — no nexus burden.
- Then consider secondary. If the condition flows from something already service-connected, § 3.310 is often cleaner than proving a fresh in-service link.
- Use direct when the condition started in service and you can show the nexus or continuity of symptoms.
- Use aggravation when a documented pre-existing condition got worse in service.
The Claim Coach is organized around exactly this logic. The first-claim track (Track A) walks direct and presumptive theories for a brand-new claim; the secondaries track (Track B) maps conditions flowing from your existing ratings. Start at the Claim Coach and it routes you to the right path based on your situation.
Quick answers
What does "service-connected" actually mean?
A condition is service-connected when the VA accepts that it is linked to your military service. That link can be established four ways: directly (the condition began in or was caused by service), secondarily (it was caused or aggravated by an already service-connected condition, under 38 CFR § 3.310), presumptively (the law presumes the link for certain conditions and service, under 38 CFR § 3.307/3.309/3.320), or by aggravation (a pre-existing condition was permanently worsened by service, under 38 CFR § 3.306). Service connection is what makes a condition compensable.
What are the three elements of direct service connection?
Under 38 CFR § 3.303, direct service connection requires three things: (1) a current diagnosed disability, (2) an in-service event, injury, or disease, and (3) a medical "nexus" linking the two. The nexus — usually a medical opinion that the current condition is "at least as likely as not" related to service — is the element that most often decides the claim.
How is secondary service connection different?
Secondary service connection (38 CFR § 3.310) links a new condition to an already service-connected one rather than directly to service. Classic examples: sleep apnea secondary to PTSD, depression secondary to chronic pain, or radiculopathy secondary to a service-connected back. You need a current diagnosis of the secondary condition and a nexus opinion connecting it to the primary service-connected disability. Aggravation of a non-service-connected condition by a service-connected one is also covered under § 3.310(b).
What is a presumptive condition?
A presumptive condition is one the VA presumes is service-connected if you meet specific service criteria — no nexus required. Examples include chronic diseases that appear within one year of separation (38 CFR § 3.309(a)), Agent Orange-related conditions for Vietnam-era veterans, Gulf War undiagnosed illnesses, radiation-related conditions, and the PACT Act burn-pit conditions under 38 CFR § 3.320. If your condition and service match a presumption, the hardest element — the nexus — is supplied by the regulation.
What is aggravation service connection?
Under 38 CFR § 3.306, if you had a condition before service that was noted at entry, and military service permanently worsened it beyond its natural progression, the increase in disability can be service-connected. There is a presumption of aggravation when a pre-existing condition worsens in service, which the VA can only rebut with clear evidence that the worsening was due to natural progression.
Which path should I use?
Use whichever fits your facts — and you can argue more than one in the same claim. If a presumption applies, lead with it (it removes the nexus burden). If your condition flows from an existing service-connected disability, secondary is often the cleanest. If the condition started in service, direct is the route. If a pre-existing condition got worse in service, aggravation applies. A strong claim often pleads alternative theories so the VA must consider each.
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Educational content only. This is not legal, medical, or financial advice. Always consult an accredited VSO or VA-accredited attorney for claim-specific guidance. CFR citations: 38 CFR § 3.303 (direct service connection and continuity of symptoms), § 3.306 (aggravation of pre-existing conditions), § 3.307 and § 3.309 (presumptive service connection, including chronic diseases and herbicide presumptions), § 3.310 (secondary service connection and aggravation), § 3.320 (PACT Act particulate-matter presumptions). The 'at least as likely as not' standard reflects the benefit-of-the-doubt rule, 38 CFR § 3.102.