The hematocrit (Hct) is 68% in a newborn. Polycythemia is defined as a venous hematocrit above 65% or a hemoglobin >22 g/dL. Polycythemia occurs in 2–4% of newborns and is rare in premature infants <34 weeks' gestation. The current definition and management of neonatal polycythemia is empirical and not evidence based. Polycythemia may result in an increase in blood viscosity, which causes a reduction in blood flow, acidosis, hypoglycemia, tissue hypoxia, and an increase in microthrombi formation. Hyperviscosity is defined as a viscosity value >14 centipoise (shear rate of 11.5 seconds) and is also defined as 2 standard deviations greater than the norm. Polycythemia hyperviscosity syndrome (PHS) is the symptom complex that involves polycythemia, hyperviscosity, and the symptoms that accompany it. Forty seven percent of polycythemic infants have hyperviscosity. Twenty-four percent of infants with hyperviscosity have polycythemia.