The combination of polycythemia and hyperviscosity in the newborn is a common problem. The incidence will vary depending on the environment and population involved because factors such a high altitude and high-risk pregnancies will markedly influence the incidence of this complication. The timing of the cord clamping at birth in a vaginally delivered infant also will make a difference with regard to the incidence of polycythemia and hyperviscosity. The pathogenesis of polycythemia and hyperviscosity is probably multifactorial and may involve events occurring during prenatal, intrapartum, and the immediate postnatal period. Polycythemia and hyperviscosity affect many organ systems. Therefore clinical manifestations are frequently nonspecific. All infants with symptomatic polycythemia and hyperviscosity should receive a partial exchange transfusion. The management of asymptomatic cases is controversial and must be individualized. The prognosis of infants with polycythemia and hyperviscosity depends heavily on the primary cause of polycythemia and hyperviscosity. Those neonates with symptoms that may or may not be related to polycythemia and hyperviscosity itself generally will have less favorable outcomes if partial exchange transfusions are not performed. The outlook of the asymptomatic polycythemia and hyperviscosity is less certain and remains to be explored.