Small cell carcinoma of the prostate is a rare disease, since only about 50 cases in the English literature and two cases in Japanese literature have been reported. Here we report a case that is the youngest ever described in the literature. A 24-year-old man was referred to our hospital with right dull lumbago and dysuria. He had the same symptom for one and half year before referral. IVP showed right non-visualizing kidney and left hydronephrosis. Form abdominal CT scans and cystoscopic findings a retrovesical tumor was highly suspicious. Transperineal needle biopsy specimens revealed an undifferentiated malignant tumor. His serum Neuron Specific Enolase (NSE) and LDH were remarkably high and whole body CT scan and upper GI tract examination demonstrated no lesion. He developed ileus and underwent exploratory laparotomy and colostomy was constructed. There was a large mass arising from the prostate which invaded into the peritoneal cavity, and multiple metastases were seen on the omentum and mesenteric lymph nodes. Specimens from the mass arising from the prostate and lymph nodes revealed small cell carcinoma pathologically. A panel of antibodies were used to seek potential tumor markers and to identify substances produced by the tumor cells including enzymes, cytoskeletal components and hormones. And stains were positive for the NSE and chronogranin. An intensive anti-cancer chemotherapy with VP-16 and CDDP was done with minor response (MR) and the serum tumor marker, LDH and NSE, decreased markedly. However, he had expired on the 58th hospital day.(ABSTRACT TRUNCATED AT 250 WORDS)