A number of studies report a high frequency of double primary cancers of the bladder and prostate. The coincidence was as high as 70% for prostate cancers in patients with bladder cancer, and 3.4% for bladder cancers in patients with prostate cancer. Two studies reviewing medical records reported a significant risk of bladder cancer after prostate cancer and of prostate cancer after bladder cancer. Only 1 of 3 cancer registry studies reported a significantly increased risk of prostate cancer after bladder cancer, and 3 of 11 studies reported a significantly increased risk in bladder cancer after prostate cancer. There was an association between DNA repair and N-acetyltransferase polymorphisms and risk of prostate and bladder cancer. These data suggest that these cancers may share a common carcinogenic process or that these patients are particularly susceptible to both cancers. Because of the association between these cancers, patients who are diagnosed with prostate or bladder cancer should be followed closely for the detection of the second urologic malignancy.