Patients with regular use of nonsteroidal antiinflammatory drugs (NSAIDs) appear to have a reduced mortality from colon cancer. As NSAID use is associated with gastrointestinal bleeding, endoscopic exploration of patients on NSAID may lead to more efficient screening and frequent detection of colon cancer. A case-control study was conducted among 12,304 veterans with a colon cancer diagnosed between 1988 and 1992. Four controls were matched by age, sex, and race to each case. The frequency distributions of previous discharge diagnoses in cases and controls were compared. Arterial embolism and thrombosis, spondylosis, peripheral vascular disease, angina, osteoarthrosis, and ischemic heart disease protected against future development of colon cancer. On the other hand, atrial fibrillation and flutter, as well as phlebitis and thrombophlebitis, were associated an increased occurrence of colon cancer after 5-10 years. The study contrasts diseases that are treated with aspirin with those that are treated with other anticoagulants. Both cause bleeding, but the reduced risk of colon cancer was seen only in conditions treated with aspirin. The difference between the two disease groups from the same VA patient population suggests that chronic use of NSAID truly protects against future development of colon cancer.