Packed red cell volume (VPRC) and total blood volume chiefly affect oxygen transport to tissues and produce the syndrome of polycythemic hyperviscosity. Several studies have demonstrated that a raised VPRC increases the blood viscosity and, under a measured pressure, decreases the flow rate in a capillary tube. Reduced flow results in a less effective tissue perfusion. The oxygen transport at different values of VPRC is shown by an inverted arc-like curve: at normal VPRC levels the oxygen transport is optimal. Hypervolemia affects oxygen transport to tissues: at the same VPRC values, hypervolemic patients have a better oxygen transport. In polycythemia vera (PV), hypervolemia may partially reduce the damage due to the increased blood viscosity. However, in some local areas where fixed vessel diameter (from arteriosclerosis) limits the increased blood flow, hyperviscosity may result in a local tissue ischemia. A relative iron deficiency with associated microcytosis due to venesection frequently occurs in PV. It is also associated with a delayed red blood cell filtration which may contribute to hyperviscosity. When thrombocytosis is present, the risk of thrombotic complications is furthermore increased. We report symptoms and signs at the onset in a series of 80 patients affected by PV. All patients have a minimum follow-up of 4 years. We also report the course of the illness, the treatment effectiveness (venesection, dibromomannitol) on clinical symptoms, the survival curve with analysis of prognostic factors at the onset and the causes of death.