To study the role of the JAK2-V617F mutation in leukemic transformation, we examined 27 patients with myeloproliferative disorders (MPDs) who transformed to acute myeloid leukemia (AML). At MPD diagnosis, JAK2-V617F was detectable in 17 of 27 patients. Surprisingly, only 5 of 17 patients developed JAK2-V617F–positive AML, whereas 9 of 17 patients transformed to JAK2-V617F–negative AML. Microsatellite analysis in a female patient showed that mitotic recombination was not responsible for the transition from JAK2-V617F–positive MPD to JAK2-V617F–negative AML, and clonality determined by the MPP1 polymorphism demonstrated that the granulocytes and leukemic blasts inactivated the same parental X chromosome. In a second patient positive for JAK2-V617F at transformation, but with JAK2-V617F–negative leukemic blasts, we found del(11q) in all cells examined, suggesting a common clonal origin of MPD and AML. We conclude that JAK2-V617F–positive MPD frequently yields JAK2-V617F–negative AML, and transformation of a common JAK2-V617F–negative ancestor represents a possible mechanism.