Among 2333 consecutive patients with myelofibrosis with myeloid metaplasia (MMM) seen at our institution, 91 fulfilled the World Health Organization (WHO) criteria for leukemic transformation (LT). All episodes of LT were myeloid in origin (acute myeloid leukemia [AML]) with all French-American-British (FAB) subtypes represented except M3; the most frequent subtypes were M7 (25.4%), M0 (22.4%), and M2 (17.9%). Cytogenetic studies during LT were available in 56 patients and revealed a clonal abnormality in 51 (91%): 30 patients had complex karyotype, 2 had core-binding factor gene lesions, and 18 had abnormalities of chromosome 5 or 7. Karyotypic evolution was documented in the majority of the patients in whom serial analysis was possible. In general, LT was fatal in 98% of the cases after a median of 2.6 months (range, 0-24.2 months). Twenty-four patients received AML-like induction chemotherapy that resulted in no complete remission: 41% reverted into chronic-phase disease and the incidence of treatment-related mortality was 33%. The remaining 67 patients received either supportive care alone (48 patients) or low-intensity chemotherapy (19 patients). Overall, survival was similarly poor in all 3 treatment categories. The outcome of LT in MMM with current therapies is dismal and either supportive care alone or appropriate clinical trials should be considered.