Primary small cell carcinoma of the prostate is a rare malignancy of unknown origin and with poor prognosis. We report a case occurring in a 74-year-old man. Radiologic findings showed that the prostate was markedly enlarged, with a lobulated contour and unclearly defined internal architecture. It was therefore impossible to distinguish it from prostatic adenocarcinoma, but the huge mass and normal range of serum tumor markers (prostate-specific antigen and prostate acid phosphatase) suggested small cell carcinoma of the prostate.