We present a case of non-Hodgkin lymphoma of the prostate referring to an 86-year-old male, who was admitted with a history of urine retention of 10 days' duration. The symptoms and signs were compatible with a diagnosis of benign prostatic hypertrophy, with no preoperative evidence of malignancy. Following transvesical prostatectomy, immunocytochemical analysis of the specimen showed a highly malignant non-Hodgkin lymphoma of the large immunoblastic B-cell type, with a cIgM (kappa) phenotype.