The JAK2^V617F mutation has been shown to occur in the overwhelming majority of patients with polycythemia vera (PV). To study the role of the mutation in the excessive production of differentiated hematopoietic cells in PV, CD19⁺, CD3⁺, CD34⁺, CD33⁺, and glycophorin A⁺ cells and granulocytes were isolated from the peripheral blood (PB) of 8 patients with PV and 3 healthy donors mobilized with G-CSF, and the percentage of JAK2^V617F mutant allele was determined by quantitative real-time polymerase chain reaction (PCR). The JAK2^V617F mutation was present in cells belonging to each of the myeloid lineages and was also present in B and T lymphocytes in a subpopulation of patients with PV. The proportion of hematopoietic cells expressing the JAK2^V617F mutation decreased after differentiation of CD34⁺ cells in vitro in the presence of optimal concentrations of SCF, IL-3, IL-6, and Epo. These data suggest that the JAK2^V617F mutation may not provide a proliferative and/or survival advantage for the abnormal PV clone. Although the JAK2^V617F mutation plays an important role in the biologic origins of PV, it is likely not the sole event leading to PV.