Although polycythemia vera is frequently encountered and has been the subject of much clinical investigation, its etiology remains obscure. We have considered the possibility that polycythemia vera is a disease characterized by an abnormality of homeostasis of the primitive cells of the reticulum. While the precise nature of the abnormality cannot be supported by experimental data, the concept has led us to examine in detail the course of polycythemia vera in a few carefully selected patients. Accelerated differentiation of stem cells is an appealing concept of the pathogenesis of polycythemia vera because of the plethora of all myeloid progeny in the blood of patients with the disease.1 Moreover, the normal appearance of the cells in the marrow suggests that there is orderly maturation, so that the defect may involve improper control in the proliferation or differentiation of primitive stem cells. Considering a defect in stem cell regulation in polycythemia