The mathematical relationship between blood viscosity and hematocrit levels was studied in 93 venous blood samples drawn within the first 6 hours of life from 20 full-term infants with weight appropriate for gestational age. A highly significant linear correlation (r = .948) between the logarithm of the viscosity at all the shear rates examined and the Hct was found. This indicates an exponential relationship between the blood viscosity and the Hct levels for every value of the Hct greater than 42%. A new dynamic definition of neonatal polycythemia, which takes into consideration the time of sampling, is suggested. By determining the mean + 2 SD, the upper limit of the normal Hct at the age of 2 hours was 71% and at the age of 6 hours was 68%.