Polycythemia vera (PV) and essential thrombocythemia (ET) are chronic myeloproliferative diseases that carry intrinsically the potential for leukemic transformation. The aims of this study were (1) to detect involvement of N-and K-ras mutations in codons 12 and 13 in the pathogenesis of the chronic and blastic phases of PV and ET, (2) to study the occurrence of microsatellite instability (MSI) in chromosomes 5 and 7 during the chronic phase and blastic transformation of the disease, and (3) to examine the incidence of leukemia in patients treated with hydroxyurea (HU). Samples of PV and ET patients were analyzed with a polymerase chain reaction. No N-or K-ras mutations were detected. A positive score for MSI in chromosome 7 was found in 1 patient with PV during leukemic transformation. Three of 69 patients developed acute myelogenous leukemia, 2 with PV and 1 with ET. As of this report, the overall incidence of leukemic transformation is 5.7% (2/35 patients) in PV and 3.3% (1/30 patients) in ET patients treated with HU. These results indicate that (1) MSI is a genetic marker that can be detected, even in a small group of patients, at the blastic phase of the disease and (2) no increased leukemogenicity was noted in this group of patients treated with HU.