The dynamic changes occurring in hematocrit and blood viscosity within the first 18 hours of life were studied in 50 full-term infants who were vaginally delivered and had weight appropriate for gestational age. In all cases, the cord was clamped within 30 seconds and cord blood was collected from the vein and artery. Subsequently, samples were taken from a peripheral vein at ages 15 minutes, and 2, 4, 6, and between 12 to 18 hours. Both the Hct and blood viscosity reach their peak at age 2 hours. The incidence of neonatal polycythemia varied greatly with age. Thus at the age of 2 hours, ten infants (20%) were polycythemic, whereas by age 6 hours only six (12%) of these infants were still polycythemic and by age 12 to 18 hours only one infant (2%) was polycythemic. A linear correlation was found between cord Hct levels and peripheral venous Hct levels by age 2 hours. None of the infants with cord blood Hct levels less than or equal to 56% had developed polycythemia, whereas ten of the 12 infants with cord Hct levels greater than 56% developed polycythemia. In this particular group of infants, cord blood Hct levels may be used for the screening of neonatal polycythemia.