Hematological malignancies can cause bone lesions, of which the most common are the punched-out foci of osteolysis seen in multiple myeloma. However, osteosclerotic lesions are more common in myeloproliferative disorders. We report the unusual case of a patient with myeloproliferative syndrome in whom the development of osteolytic lesions revealed transformation to acute leukemia. In 2007, this 82-year-old man with essential thrombocythemia since 1994 developed primary polycythemia with the JAK2 mutation V617F. In July 2017, he was evaluated for an osteolytic lesion in the right humerus with incipient fracturing. Bone marrow smear results provided only limited information, due to the myelofibrosis, showing 7% of blast cells with no plasmacytosis. No solid malignancies were identified by imaging studies. Examination of a right humeral biopsy specimen taken during internal fixation showed myeloproliferative syndrome with osteosclerosis and grade-3 myelofibrosis, as well as a malignant proliferation of large cells carrying the leukocyte cluster of differentiation antigens CD45, CD 43, CD4, and CD34. The diagnosis was transformation of the myeloproliferative syndrome to acute myeloid leukemia. The development of an osteolytic lesion during the course of myeloproliferative syndrome is an exceedingly rare event that should suggest acute leukemic transformation.