OBJECTIVE: To examine the relationship between daily aspirin use and risk of prostate cancer in a large, racially diverse cohort of men followed for up to 32 years. METHODS: The study population included 90,100 male subscribers to the Kaiser Permanente Medical Care Program who had received one or more multiphasic health checkups between 1964 and 1973. This general health checkup included a self-completed questionnaire that requested men to record if they took more than six aspirin almost every day during the previous year. Subjects were followed for the development of prostate cancer using the local tumor registry. Cox regression was used to estimate relative risks (RR) and 95% confidence intervals (CI). RESULTS: A total of 2,574 men developed prostate cancer. Of these, 1617 had local stage disease and 719 had either regional or distant disease at diagnosis. A total of 2466 men (2.7%) reported taking more than six aspirin almost every day during the past year at one or more health checkups. After adjusting for birth year, education, race, and the number of health checkups. the relative risk of prostate cancer associated with this amount of aspirin use was 0.76 (95% CI 0.60-0.98). Relative risks did not differ by race and were similar for both local stage and regional or distant stage prostate cancer. CONCLUSION: Results from our large, multiracial cohort study support a modest inverse relationship between daily consumption of more than six aspirin and prostate cancer risk.