A case-control study was conducted within Group Health Cooperative of Puget Sound to evaluate the relation between prior vasectomy and the risk of prostate cancer. Cases consisted of 175 men newly diagnosed with histologically confirmed prostate cancer during 1989-1991. A total of 258 controls, matched to cases on birth year and membership status, were randomly selected from the general membership of the plan. Information was collected from mailed questionnaires and medical records on medical history, including prior vasectomy, anthropometric measures, family history of prostate cancer, personal habits, and medical care utilization, and demographic factors. Conditional logistic regression analyses showed that the odds ratio for prostate cancer associated with vasectomy was 0.86 (95% confidence interval 0.57-1.32) after adjustment for confounders. The odds ratio estimate did not differ substantially by age at vasectomy or time since vasectomy. However, the odds ratio estimate for prostate cancer associated with vasectomy tended to be increased among men who had a father or brother with prostate cancer. Nevertheless, the increased risk may be related to detection bias or differential participation rates due to both vasectomy status and a family history of prostate cancer. These results suggest no overall association between vasectomy and prostate cancer.