We investigate the possible reasons for the co-occurrence of bladder and prostate cancer and discuss its clinical relevance. The co-occurrence of bladder and prostate cancer can (partly) be explained by a survival effect and diagnostic bias. Radiotherapy for prostate cancer might be responsible for a significant increase in secondary bladder cancers, with an absolute effect that is small. Existing international consortia should expand to include secondary primary cancers as well as the index tumor. This will facilitate the investigation of common etiology and will help to identify subgroups that are susceptible to treatment-induced carcinogenesis. The prognosis after a diagnosis of both bladder and prostate cancer seems to depend mainly on the prognosis of the bladder tumor.