Megakaryocytopoiesis in polycythemia vera (PV) was characterized by in vitro growth of marrow megakaryocytic progenitors (CFU-Meg) and quantitation of megakaryocyte numbers in marrow biopsy specimens in 14 patients with PV. Megakaryocyte numbers and CFU-Meg numbers in the 14 patients were variable, with values from the control range to markedly increased numbers. Nine of the 14 patients showed spontaneous CFU-Meg growth, and the presence or absence of spontaneous CFU-Meg growth was found to be related to the degree of marrow megakaryocyte increase, i.e., those with spontaneous CFU-Meg had higher megakaryocyte numbers. None of the plasmas from the PV patients contained detectable levels of megakaryocyte colony-stimulating activity (Meg-CSA), assayed using the culture with nonadherent normal marrow cells and tested plasma without phytohemagglutinin-stimulated leukocyte-conditioned medium as an exogenous source of Meg-CSA. Increased megakaryocyte numbers and resultant thrombocytosis in PV are likely based on the abnormal population of CFU-Meg.