In a series of 218 subjects diagnosed as having nonleukemic chronic myeloproliferative disorders in a single institution within a 18-year period, 13 instances of acute transformation were registered. They corresponded to 8 out of 70 patients with idiopathic myelofibrosis (IM), 4 out of 91 with polycythemia vera (PV), and 1 out of 57 with essential thrombocythemia (ET). The actuarial probability of developing such a complication at 100 months from diagnosis reached 20.6% in IM, 8.7% in PV, and 4% in ET. Only 1 IM patient whose condition developed into acute leukemia had received prior cytolytic therapy, whereas, in contrast, all PV and ET patients showing this pattern had previously been treated with either ³²P or alkylating agents. On the other hand, acute transformation in IM generally had an insidious presentation, contrasting with its abrupt onset in most PV and ET patients. Most acute leukemias (12 out of 13) exhibited a myeloid phenotype. The patients’ median survival from diagnosis of the acute transformation was only 3 months, the development of this complication significantly shortening the patients’ overall survival.