Because small cell anaplastic carcinoma of the prostate is an uncommon tumor, it has remained a poorly defined entity. To elucidate further the clinical, pathological and immunohistochemical characteristics of this cancer the 27 patients who presented to the Mayo Clinic from 1960 to 1990 were reviewed. Of these patients 18 (67%) presented with pure small cell anaplastic carcinoma, and 9 (33%) were diagnosed with small cell anaplastic carcinoma and adenocarcinoma of the prostate. Twenty-six patients (96%) had either stage C or D disease at the time of diagnosis. Two patients presented with a paraneoplastic syndrome, including 1 man with inappropriate antidiuretic hormone secretion and 1 who suffered from thyroxine intoxication. Of 24 men with long-term followup 22 (92%) died of small cell anaplastic carcinoma of the prostate despite antiandrogen therapy and the remaining 2 are alive with active, progressive disease. The median survival time following diagnosis was 17.1 months (range 2 to 90 months). All tumors with tissue available for immunohistochemical staining reacted positive for neuron-specific enolase, indicating that small cell anaplastic carcinoma of the prostate is most likely a neuroendocrine neoplasm. No tumor stained positive for either prostatic acid phosphatase or prostate specific antigen. Pathologically, small cell anaplastic carcinoma of the prostate appears to be similar to oat cell carcinoma of the lung. This series of 27 patients emphasizes that small cell anaplastic carcinoma of the prostate is highly malignant, is frequently of advanced stage at presentation, responds poorly to antiandrogen therapy and has a poor prognosis.