The incidence of prostatic disease (prostatic cancer and benign prostatic hypertrophy) was studied in a cohort of 5,332 vasectomized Kaiser Permanente Health Plan members, each matched with 3 nonvasectomized comparison subjects. Relative risks of 1.0 for prostatic cancer (68 cases, 95 per cent confidence limits 0.6 and 1.7) and 1.2 for benign prostatic hypertrophy (90 cases, 95 per cent confidence limits 0.7 and 1.8) were associated with vasectomy. In this study group the risk of prostatic disease was not altered by a history of vasectomy.