One percent to 5% of all newborns in the United States are polycythemic.As the venous hematocrit rises above 65%, the thickness or viscosity of whole blood also increases, potentially compromising blood flow to a variety of organs. Fortunately, relatively few infants who have neonatal polycythemia or hyperviscosity develop complications attributable to their thick blood; however, controversy and the need for continued research envelop the issue of which infants are at risk and need to be treated. This article reviews the differential diagnosis, clinical presentation, and treatment of neonatal polycythemia.