Americans 50 years of age and older are advised to test their stools for occult blood to detect colorectal neoplasms. Many will be taking 1,300 mg of aspirin daily because of cerebrovascular disease or smaller amounts for cardiovascular disease. To determine if 1,300 mg of aspirin causes positive hemoccult II tests, 27 healthy volunteers ate a red meat-free, high-fiber diet. Their stools were negative for occult blood during a 3-day control period and remained negative while they took 1,300 mg aspirin daily for an additional 7 days. This indicates that 1,300 mg aspirin daily for 1 week does not cause positive hemoccult II testing. Those taking this dose of aspirin probably need not interrupt therapy to perform hemoccult II testing.