The JAK2 617V>F mutation is frequent in polycythemia vera (PV) and essential thrombocythemia (ET). Using quantitative polymerase chain reaction (PCR), we found that high levels of JAK2 617V>F in PV correlate with increased granulocytes and high levels of hemoglobin and endogenous erythroid colony formation. We detected normal progenitors and those that were heterozygous or homozygous for the mutation by genotyping ET and PV clonal immature and committed progenitors. In PV patients, we distinguished homozygous profiles with normal, heterozygous, and homozygous progenitors from heterozygous profiles with only heterozygous and normal progenitors. PV patients with a heterozygous profile had more mutated, committed progenitors than did other PV and ET patients, suggesting a selective amplification of mutated cells in the early phases of hematopoiesis. We demonstrated that mutated erythroid progenitors were more sensitive to erythropoietin than normal progenitors, and that most homozygous erythroid progenitors were erythropoietin independent. Moreover, we observed a greater in vitro erythroid amplification and a selective advantage in vivo for mutated cells in late stages of hematopoiesis. These results suggest that, for PV, erythrocytosis can occur through two mechanisms: terminal erythroid amplification triggered by JAK2 617V>F homozygosity, and a 2-step process including the upstream amplification of heterozygous cells that may involve additional molecular events.