A 46-year-old man with Werner's syndrome was admitted with epigastralgia and body weight loss. The peripheral blood findings showed anemia, thrombocytosis and eosinophilia. Bone marrow aspiration and biopsy revealed increases in eosinophils and megakaryocytes, myelodysplastic change with 6.6% myeloblast, and myelofibrosis. Chromosomal analysis revealed 46, XY, +der(1;7), -7, del(20). He was diagnosed as having myelodysplastic syndrome with myelofibrosis or essential thrombocythemia. Three months later, pancytopenia appeared with a relative increase of blasts positive for CD41 and negative for myeloperoxidase. He died of respiratory failure due to pneumonia. An autopsy revealed severe myelofibrosis with proliferation of megakaryocytes and blasts. A final diagnosis of acute megakaryoblastic leukemia was made. Werner's syndrome is rare, and it is even more unusual to have the complication of acute leukemia with der (1;7) seen in this case.