VA secondary service connection: what 3.310 says.

The short version

"Secondary service connection" is the common name for what 38 CFR 3.310 does. The regulation recognizes that one medical problem can lead to another. If a condition that VA has already service connected causes a new condition, or makes an existing unrelated condition worse, the new or worsened condition can itself be service connected. This page follows the text of the regulation as published in the eCFR, current through October 29, 2026, and does not predict how any case will turn out.

Paragraph (a): causation

Paragraph (a) is the general rule. Except as provided in 3.300(c), a disability "which is proximately due to or the result of a service-connected disease or injury shall be service connected." Two things follow from the wording. First, the starting point is an existing service-connected disease or injury; secondary connection builds on something VA has already granted. Second, "proximately due to or the result of" is a causation standard. A condition that merely appears alongside the service-connected one, without being caused by it, does not meet it.

The paragraph ends with a rule about how the pieces are treated: once service connection is established for a secondary condition, "the secondary condition shall be considered a part of the original condition." That wording matters for how the two conditions are viewed together, and the rating schedule in 38 CFR Part 4 then governs how each is evaluated.

Paragraph (b): aggravation and the baseline rule

The second route is aggravation. Paragraph (b) says any increase in severity of a nonservice-connected disease or injury that is proximately due to or the result of a service-connected condition, and not due to the natural progress of the nonservice-connected disease, will be service connected. A plain-language example: a veteran has a knee disability that is service connected, and a pre-existing, unrelated back condition gets worse because the veteran compensates for the knee. The worsening that is caused by the knee, as opposed to the back condition's own natural course, is what the paragraph reaches.

The regulation then adds a condition that is easy to miss. VA "will not concede" aggravation unless the baseline level of severity of the nonservice-connected condition 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. In other words, the file needs a before-picture. Without a baseline, there is nothing to measure the increase against.

The paragraph also describes the arithmetic of the result. The rating activity determines the baseline and current severity under the Schedule for Rating Disabilities, then determines the extent of aggravation by deducting the baseline, as well as any increase due to natural progress, from the current level. Only the remainder is the aggravation.

A worked example of the deduction

The regulation describes this as a subtraction, so here is one with invented numbers purely to show the method. Suppose a condition would be rated 10 percent at its baseline, 40 percent today, and the evidence shows that 10 points of that increase are due to the condition's natural progress. The deduction in paragraph (b) removes the baseline (10) and the natural progress (10) from the current level (40), leaving 20 points attributable to the service-connected condition. The numbers are illustrative only. Real rating decisions use the Part 4 criteria for the specific condition, and VA, not this page, makes the finding.

Paragraph (c): cardiovascular disease after amputation

Paragraph (c) is a built-in presumption. Ischemic heart disease or other cardiovascular disease developing in a veteran who has a service-connected amputation of one lower extremity at or above the knee, or service-connected amputations of both lower extremities at or above the ankles, "shall be held to be the proximate result" of the amputation or amputations. No separate causation opinion is needed when those facts are present.

Paragraph (d): traumatic brain injury

Paragraph (d) works the same way for a veteran with a service-connected traumatic brain injury (TBI). In the absence of clear evidence to the contrary, these are held to be the proximate result of the TBI:

• Parkinsonism, including Parkinson's disease, following moderate or severe TBI.
• Unprovoked seizures following moderate or severe TBI.
• Presenile dementia of the Alzheimer type, frontotemporal dementia, and dementia with Lewy bodies, if manifest within 15 years following moderate or severe TBI.
• Depression, if manifest within 3 years of moderate or severe TBI, or within 12 months of mild TBI.
• Diseases of hormone deficiency that result from hypothalamo-pituitary changes, if manifest within 12 months of moderate or severe TBI.

These time limits and severity levels are not walls. Paragraph (d)(2) says they do not preclude service connection for conditions shown by evidence to be proximately due to a service-connected TBI, and a claim that misses them is developed and decided under the generally applicable principles of service connection.

Paragraph (d)(3) defines severity with a table. Mild means normal structural imaging, loss of consciousness of 0 to 30 minutes, alteration of consciousness up to 24 hours, post-traumatic amnesia up to 1 day, or a Glasgow Coma Scale score of 13 to 15. Moderate means loss of consciousness over 30 minutes and under 24 hours, alteration of consciousness over 24 hours, post-traumatic amnesia over 1 and under 7 days, or a score of 9 to 12. Severe means loss of consciousness over 24 hours, post-traumatic amnesia over 7 days, or a score of 3 to 8. Severity is judged from symptoms at the time of injury or shortly after, not from current functioning, and a TBI meeting criteria in more than one category is ranked at the highest level in which a criterion is met.

What this page does not cover

This page covers 3.310 only. It does not cover the general rules for direct service connection in 3.303, which the regulation cross-references and which this page did not read, or how a secondary condition is rated. It does not tell you whether your condition is secondary to another, which needs medical evidence and a VA decision.