A four-year-old boy suffered from acute myelofibrosis which became acute lymphoblastic leukemia during the course of the disease. Histologic and immunophenotypic studies revealed that the leukemic cells were lymphoblasts. Immunophenotypic examination also excluded the possibility of acute megakaryoblastic, monocytic and myeloid leukemia. Thus terminal leukemic transformation, in some cases of acute myelofibrosis, may be of a lymphoid nature.