A case-control study was conducted between 1957 and 1965 on 128 patients with benign prostatic hyperplasia (BPH), 256 age-matched controls, 290 prostate cancer patients and 290 age-matched controls for the prostate cancer patients, all of whom had completed the Roswell Park Memorial Institute epidemiology questionnaire and were interviewed on admission to the Institute. Compared to the control groups a higher proportion of both case groups were Protestants and residents of smaller towns. The major finding in this case-control study was a significantly higher risk for prostate cancer in fertile males compared to both married and non-married, but infertile males. This finding was confirmed when fertility was used as a variable for the classification of study groups in an earlier prospective study reporting the follow-up of patients with BPH and non-neoplastic controls. In this study, patients with children were found to have a relative risk of 2.69 for prostate cancer compared to the married patients with no children. Fertility may be a manifestation of constitutional-hormonal factors that increase the risk of prostate cancer.