In March 1990, a 61-year-old man was given a diagnosis of essential thrombocythemia with a normal karyotype and subsequently treated with hydroxyurea. In November 1995, he underwent surgery for gastric cancer with thereafter received tegafur/uracil for 2 years. Refractory anemia with excess of blasts in transformation and chromosomal abnormalities including -5, -7, 20q-developed in August 1998. Combined chemotherapy with daunorubicin, cytarabine, mercaptopurine, and prednisolone, had only limited effectiveness. Acute myeloid leukemia was finally diagnosed in October 1998, and chromosomal analysis disclosed inv(3) in addition to -5 and -7. The appearance of inv(3) might be related to leukemic transformation of hematopoietic stem cell disease with an increase in the number of megakaryocytes and platelets.