Klinefelter syndrome (KS) is a chromosome abnormality characterized by a 47, XXY 
karyotype associated with hypogonadism and infertility. We present two cases of 
leukemia in patients with KS. The first patient presented with acute 
promyelocytic leukemia. He relapsed after the end of treatment. The second 
patient was diagnosed with chronic myeloid leukemia. Treatment with imatinib 
failed and the patient presented with myeloid blast crisis.