One way to strengthen a VA claim that is easy to overlook is the secondary condition. A disability the VA has already connected to your service can cause, or worsen, other conditions, and those follow-on conditions can be service connected too. The rule that governs this is 38 CFR 3.310, and it comes in two flavors that are easy to mix up: causation and aggravation. Getting the difference right is what lets you frame the claim correctly.
What secondary service connection actually is
Start with the plain-language version the VA uses. A secondary claim, in the VA's words, is for "a new disability that's linked to a service-connected disability you already have." The VA's own example is arthritis that develops because of a service-connected knee injury. VA describes secondary claims as "claims for disabilities that developed as a result of or were worsened by another service-connected condition."
So the structure is always the same. You already have a primary condition the VA connected to your service. Then a second condition shows up that is tied to that primary. The second condition is what you claim as secondary. It does not have to trace back to an in-service event on its own, because its bridge to service runs through the primary condition.
Causation: proximately due to your service-connected condition
The first flavor is causation, set out in 38 CFR 3.310(a): "disability which is proximately due to or the result of a service-connected disease or injury shall be service connected." The key phrase is "proximately due to or the result of." If your service-connected condition is the proximate cause of a new condition, the new condition is service connected in full.
This is the cleaner of the two. The new condition exists because the primary condition brought it on. Think in terms of a chain: the service-connected primary is the reason the secondary developed at all.
Aggravation: when service connection makes an existing condition worse
The second flavor is aggravation, and it is more technical because the VA has to separate the part it will pay for from the part it will not. Under 38 CFR 3.310(b), "VA will not concede that a nonservice-connected disease or injury was aggravated by a service-connected disease or injury unless the baseline level of severity of the nonservice-connected disease or injury is established by medical evidence created before the onset of aggravation or by the earliest medical evidence created at any time between the onset of aggravation and the receipt of medical evidence establishing the current level of severity of the nonservice-connected disease or injury."
That is a long sentence, so here is what it is doing. Aggravation covers a condition that was not caused by service, but that a service-connected condition made worse. The VA does not treat the whole thing as service connected. It measures how bad the condition was before the aggravation started, calls that the baseline, and then looks at how much worse it got. To do that, it needs medical evidence of the baseline. Without a documented baseline, the regulation says the VA will not concede aggravation.
| Concept | Rule | What the VA connects |
|---|---|---|
| Causation (3.310(a)) | New condition is proximately due to or the result of a service-connected condition | The new condition, as service connected |
| Aggravation (3.310(b)) | A nonservice-connected condition is made worse by a service-connected condition | The increase above a documented baseline |
What you still have to prove
Secondary service connection is powerful, but it is not automatic. Having a service-connected primary condition does not turn every future health problem into a granted secondary claim. You still need a current diagnosis of the secondary condition, and you still need a medical link connecting it to the primary. That link is usually a medical opinion, sometimes called a nexus, explaining how the primary condition caused or aggravated the secondary one. Learn how those pieces fit together in VA Service Connection Evidence 101, and see which door a secondary claim goes through in VA Claim Types Explained.
Why secondary claims get missed
Secondary conditions are easy to miss, because they can surface years after the primary condition was granted, and by then the connection is not obvious. A joint injury changes how you move, which stresses other joints. A chronic condition changes your activity, your sleep, or your medication, which can affect other systems over time. None of that is automatic, and this guide is not saying any particular pairing will be granted. The point is narrower and more useful: when a new health problem appears after a service-connected condition, it is worth asking whether the two are medically linked, because if they are, secondary service connection is the doorway.
The evidence job has the same shape as any claim. You show a current diagnosis of the secondary condition, and you provide a medical opinion connecting it to the primary. Under the causation rule, that opinion explains how the primary proximately caused the secondary. Under the aggravation rule, it also has to speak to the baseline, meaning how severe the secondary condition was before the aggravation began, because the VA pays for the increase above that baseline. A clear medical opinion is the piece that ties the secondary condition to the primary, and you can read what those opinions do in VA DBQ and Nexus Letters Explained.
Hypothetical example. Consider a veteran we will call Rosa with a service-connected knee injury. Years later, favoring that knee changes how she walks, and she develops a hip problem. If the medical evidence shows the hip condition is proximately due to the knee condition, that is a causation theory under 3.310(a). If instead Rosa already had mild hip arthritis that the knee condition then made worse, that is an aggravation theory under 3.310(b), and the VA would look for medical evidence of how bad the hip was before the aggravation began. Same veteran, two different regulatory paths, depending on the facts and the evidence.
What this does not mean
A service-connected condition does not make every later condition automatically secondary, and filing a secondary claim does not guarantee it will be granted. You still need a current diagnosis and a medical link, and under the aggravation rule you also need evidence of the baseline. Secondary service connection is a theory you support with evidence, not a switch that flips on by itself.
Where to go next
To see common primary-to-secondary pairings and how they are typically documented, explore our Secondary Conditions explorer and browse the conditions library. When you are ready to file, follow the ordered steps in How to File a VA Disability Claim, and estimate how a new rating could combine with your existing ones using the combined rating calculator. Learn any unfamiliar term in the glossary. We educate and help you organize your evidence. We never prepare or file your claim.