A patient is described who presented initially with benign prostatic hypertrophy and urinary retention. He underwent a prostatectomy and subsequent bilateral orchiectomy for microscopic "carcinoma of the prostate." Leydig cell hyperplasia was found on pathologic examination of the testes and evaluation for Klinefelter's syndrome was initiated. He was studied with buccal smears, polymorphonuclear leukocyte examinations, hormone excretion titers, short-term leukocyte tissue cultures with chromosome analyses, and, finally, by autopsy. He was proved to have Klinefelter's syndrome, benign prostatic hypertrophy, and, at autopsy, malignant carcinoid with widespread metastases; he did not have carcinoma of the prostate. The occurrence of this case raises many questions about the hormonal make-up of the Klinefelter's patient, about the stimulus for development of benign prostatic hypertrophy, and about the coincidental occurrence of solid tumors and chromosomal anomalies.