Increased viscosity of the blood in newborn infants has been known to be associated with significant morbidity for more than a decade.1-4 A recent review of neonatal polycythemia and hyperviscosity by Black and Lubchenco5 summarizes the available literature and emphasizes the diagnostic and management problems that pediatricians currently face. This commentary will attempt to highlight some of these problems. During this discussion, measurements of hematocrit refer to venous microhematocrits and viscosity refers to measurements by the Wells-Brookfield microviscometer on heparinized whole blood samples, unless otherwise specified. The syndrome of neonatal hyperviscosity may be defined as alterations in flow properties of blood associated with symptoms and signs of organ dysfunction.