Purpose: We evaluated the importance of positive family history, age at diagnosis and history of vasectomy in predicting the risk for prostate cancer in the brothers of prostate cancer patients. Materials and Methods: A total of 1,084 men with newly diagnosed prostate cancer responded by interview to a family history survey, which included detailed information on the diagnosis of any cancer in the parents of the proband, diagnosis of prostate cancer in male relatives and age at onset of prostate cancer in the proband. A history of vasectomy was also obtained from the proband. The control cases consisted of 935 spouses of the probands who were administered the same questionnaire in an identical fashion. Results: Prostate cancer was not significantly associated with other types of cancer in proband parents. The presence of prostate cancer in the father, grandfather or uncle of the proband significantly increased the risk of prostate cancer in proband brothers. Early age at onset in the proband was also associated with an increased risk to the proband brothers. Conclusions: Men with a family history of prostate cancer are at a significantly increased risk for prostate cancer, especially if the affected relative had early onset of cancer. Prostate cancer does not seem to be associated with a higher incidence of other cancers in family members.