A terminally ill veteran of the mid-Pacific atomic bomb tests has finally won a five-year fight to get official government recognition of a cause-and-effect relationship between fallout more than 20 years ago and the cancer that is killing him today.

In a 22-page decision issued over the Thanksgiving holiday weekend, the Board of Veterans Appeals overruled earlier findings by Veterans Administration disability boards in the case of Orville Kelly, 49, of Burlington, Iowa.

The board decided that Kelly is entitled to "service connection for a malignant reticuloendothelian process" (a cancer of the lymphatic system) and warrants benefits as a disabled veteran.

Kelly, who started a nationwide self-help organization for radiation-induced cancer victims called "Make Today Count," was in San Francisco on a speaking tour when he learned of the ruling. He expressed gratification and predicted the decision would help other veterans who received radiation overdoses during the era of U.S. atmospheric bomb testing in the 1960s.

Fred Mullen, an appeals consultant with the Veterans of Foreign Wars here, said nearly 800 radiation-connected claims have been filed with the VA. Only eight in which cancer was one of the bases for the claim have been allowed, and of these there were only three -- including Kelly's -- where cancer was the only disorder alleged.

Until recently, veterans seeking service-connected status for cancer were confronted by a classic "Catch 22." The VA generally took the position that unless symptoms of a condition showed within the first year after separation from service, the disability would not be allowed as service- connected. But cancer usually does not show up for many years: in Kelly's case 15.

Testimony by Kelly and other cancer-disabled veterans at a hearing of the Senate Veterans Affairs Committee last May led to a liberalization of this policy, according to Mullen. Two cases similar to Kelly's were decided favorably this summer, but in one of the two cases the veteran died of his cancer before the decision could be handed down.

The Board of Veterans Appeals noted in its decision that it would be impossible to positively link the radiation Kelly received with the cancer he subsequently developed. However, the board stated:

"The Veteran was exposed to an indeterminate amount of ionizing radiation in service while stationed in relative proximity to more than 20 nuclear explosions over several months in 1957 and 1958 . . . The radiation exposure was a probable factor in the development of his malignancy."