A retrospective morphologic investigation was conducted to determine the incidence of concurrent primary carcinomas of the urinary bladder and prostate gland and to identify their patho-histologic features using transurethral resection specimens. The study was based upon 1281 cases with the initial diagnosis of a benign or malignant epithelial or mesenchymal tumor of the bladder. In 294 of the patients (23.0%), transurethral resection specimens were also available from the prostate. The incidence of double carcinomas (n = 52) was found to be 4.2% of all vesical carcinomas reviewed (n = 1228) and 17.7% of only those cases with biopsies available from both organs. Of the coexisting bladder carcinomas, 81% represented papillary transitional cell carcinomas grades 1 and 2. These were associated in 78.6% of the cases with well and moderately differentiated carcinomas (grades 1 and 2) of the prostate gland, showing either a uniform glandular or a pluriform glandular and cribriform pattern of growth. Half of the concurrent cancers of the bladder were noninvasive and 19.2% had infiltrated only the lamina propria. Thus, the great majority of coincidental vesical and prostatic carcinomas proved to be well and moderately differentiated and by far the majority of the bladder carcinomas exhibited a low stage. Since there was no typical association of particular histologic carcinoma types coexisting with each other and because the extent of local spread was unremarkable, it is impossible to predict the occurrence of double carcinomas of the urinary bladder and prostate on morphologic grounds.