This is a case report of a 60-year-old male patient with essential thrombocythemia (ET) that progressed to acute myeloid leukemia (AML) in approximately 9 years. His platelet count decreased approximately 8 years after ET treatment with hydroxyurea (HU) and aspirin. The dose of HU was reduced because of suspected myelosuppression due to HU; however, myelosuppression did not improve. Bone marrow examination revealed myelofibrosis; therefore, ruxolitinib was administered. Approximately 1 year later, his leukocyte and blast counts in the peripheral blood increased; thus, ET was judged to have progressed to AML-myelodysplasia-related change. Induction chemotherapy and consolidation therapy were initiated; however, the patient unfortunately failed to achieve complete remission. We then continued to administer salvage chemotherapy; however, his general condition worsened, and he died from cerebral hemorrhage. The karyotype at the onset of ET was 46,XY, which changed to 47,XY,del(7q),+8 at the time of AML diagnosis. In addition, genetic testing revealed FLT-3 ITD mutation. His histopathological analysis showed subarachnoid and intraparenchymal hemorrhages and tumor cell infiltration into the cerebrum, brainstem, and cerebellum. In this case, deletion of the long arm of chromosome 7, additional abnormalities in chromosome 8, and FLT3-ITD mutation were confirmed as risk factors for having developed secondary AML for approximately 9 years and death from cerebral hemorrhage 1 year later.