Cancer of the prostate was studied among 838 patients with benign prostatic hyperplasia and 802 age-matched controls. Patients were followed for an average of 10.7 years and controls for 11.2 years. Twenty-four patients and 26 controls subsequently developed prostate cancer. Patients with prostatic hyperplasia were not at increased risk for prostatic cancer. No significant differences between patients and controls were found in marital status or in the frequency of bladder cancer, diabetes, drug-use history, family medical history, and smoking or drinking habits.