We reported the case of a 35-year-old man with Klinefelter's syndrome and a malignant Leydig cell tumor of the testis. Bilateral gynecomastia and right testicular enlargement led the patient to seek medical assistance. Despite initial orchidectomy two years later the patient developed lung and iliac lymph node metastases. The tumor appeared to be refractory to chemotherapy and to hormonal treatments including op'DDD. Finally, the patient died within 20 months of developing metastases. Leydig cell tumor is an exceedingly rare tumor, especially when associated with Klinefelter's syndrome. This association as well as presentation, pathologic features, hormonal abnormalities, clinical course and response to therapy of malignant Leydig cell tumors are discussed.