Reported is the second documented case in Japan of acute leukemia with Klinefelter's syndrome. The patient, a 52-year-old male, was admitted to hospital on July 9, 1983, after complaining of a fever and general malaise. The hematological examinations revealed a WBC count of 14,300/cmm with 32% being leukemic cells. Bone marrow aspiration disclosed a nucleated cell count of 687,000/cmm with 41.6% being leukemic cells consisting of myeloblasts. The endocrinic laboratory data showed a high level of urine gonadotropin and chromosomal studies revealed a 47,XXY karyotype. A diagnosis of acute myelogenous leukemia (M1) with Klinefelter's syndrome thus was derived from these findings.