A 54-year-old female patient developed acute erythroleukemia after an 8-year course of primary myelofibrosis. The latter harbors the JAK2-V617F mutation and was treated with hydroxyurea and anagrelide. A bone marrow trephine biopsy disclosed 2 morphologically distinct areas of chronic primary myelofibrosis and acute erythroleukemia. Microdissection and a separate molecular pathological analysis was performed. Although the activating JAK2-V617F mutation was not maintained in blasts of acute erythroleukemia, it was detectable in the chronic phase of primary myelofibrosis, indicating that this mutation did not play a role in the leukemic transformation of erythroid cells.