Carcinosarcoma of the prostate is a biphasic tumor containing adenocarcinoma (ACA) and recognizable sarcomatous components. It is a rare neoplasm with only 12 previous reported cases. We describe three additional cases arising between 4 and 6 years after initial diagnosis of prostatic ACA. Two patients were initially treated by prostatectomy, pelvic external beam radiotherapy, and hormonal manipulation. The third patient was treated by pelvic lymphadenectomy and 125I implants. After the development of the sarcomatous component, the first two are still alive with distant metastases and residual pelvic disease at 9 and 17 months. The third patient died with disease 7 months after diagnosis. Histologically, prostatic ACA was recognized in all three cases, as well as a neoplastic mesenchymal component that appeared later. Foci of osteosarcomatous, chondrosarcomatous, and myosarcomatous differentiation were recognized in two of the three cases. Based on the chronologic and the histologic evolution of the neoplasm, we favor sarcomatoid transformation of the ACA as the most likely histogenesis. It appears that radiotherapy and hormonal therapy may be important in the development of at least some of these tumors.