Klinefelter's syndrome is characterized by abnormal karyotype 47, XXY and a phenotype associated with hypogonadism and gynecomastia. Often the disease can be diagnosed accidentally, when carrying out cytogenetic analysis in cases of a malignant blood disease. We present the clinical case of a patient diagnosed with acute myelomonoblastic leukemia-M4 Eo (AML- M4), where by means of classic cytogenetics a karyotype was found corresponding to Klinefelter's syndrome. Three induction courses of polychemotherapy wermade, which led to remission of the disease, documented both flowcytometrically and cytogenetically.