Polycythemia vera (PV) is a clonal disorder characterized by trilinear hematopoietic proliferation. PV progenitors are hypersensitive to several growth factors although their receptor expression is reduced or absent. In this study selected CD34+ peripheral blood (PB) cells from PV patients, PB healthy donors and patients with secondary polycythemia (SP) were investigated and compared concerning frequency, morphology, antigen expression, transcription of differentiation markers and proliferation as well as apoptosis rate following short-term culture. The highest amount of CD34+ cells was found in the SP group while the frequency was slightly lower but more variable in PV. Native PV CD34+ cells varied in shape and form and developed intracytoplasmatic organelles like mitochondria and a more extended Golgi apparatus while in the other groups they constituted a uniform phenotype. However, no premature transcripts for glycophorin A (GypA) and CD41b, both markers for advanced erythropoiesis and megakaryopoiesis, could be detected in sorted PV progenitors. Also, coexpression for early acting hematopoietic cytokine receptors IL-3Ralpha and c-Kit and for initial erythropoiesis (GypC) or megakaryopoiesis (CD61) was similar in the different groups. Performing 96 h cocultures with bone marrow (BM) fibroblasts the frequency of CD34+ cells was elevated, downregulation of IL-3Ralpha delayed, coexpression of GypC reduced and proliferative activity higher in the PV group. Our results suggest that primitive hematopoiesis is altered in PV because PB CD34+ cells in this disease are characterized by a maturation dissociation with increased activation in untreated populations and a delayed differentiation in short-term cultures.