A unique case of carcinosarcoma of the prostate in a 66-year old man is presented. The patient underwent a total prostatectomy under the diagnosis of prostatic carcinoma. Despite adjuvant radiotherapy, the patient died about 3 months after the operation. The surgically resected primary tumor was composed of two histologically distinct elements, an adenocarcinoma and sarcoma with various mesodermal components. These elements were intimately intermingled and transitional forms were often noted. The sarcomatous portion contained areas of chondromatous differentiation. The pathogenesis of this complex neoplasm is discussed.