Clinical information and histological slides of 20 cases of small cell carcinoma of the prostate seen at The University of Texas M. D. Anderson Hospital and Tumor Institute at Houston over a 23-year period were reviewed. Patient's ages ranged from 30 to 89 years (median, 67 years). In nine cases, pure adenocarcinoma of the prostate preceded recognition of the small cell component by 7 months to 8 years (median, 18 months); five of these were initially at Stage A. There was a small cell component at presentation in 11 cases (10, Stage D). Small cell carcinoma was merging with the adenocarcinoma in 11 cases and represented 30% to 90% of total tumor volume. Eleven of 20 patients died of their disease. Those presenting initially with a pure adenocarcinoma survived between 7 months and 9 years (median, 24 months). After the recognition of the small cell carcinoma component, regardless of a prior history of adenocarcinoma, death followed within 1.5 years (median, 5 months). This study suggests a biologic difference in behavior in prostatic carcinoma containing a small cell carcinoma component. The small cell component may manifest early or late in the disease.