This prospective study was designed to test the correlation of blood volume per kilogram of body weight with birthweight in polycythemic neonates. One hundred and sixty-four neonates with venous hematocrits of 65% or greater were treated with partial exchange transfusion. Follow-up venous hematocrits were obtained, and BV/kg was estimated for each infant based on the exchange volume used and the observed change in hematocrit. Birth weight ranged from 1,210 to 5,080 gm. Individual blood volume estimates ranged from 44 to 176 ml/kg, with a mean of 88.4 +/- 23.5 ml/kg. There was a highly significant negative linear correlation of BV/kg with birth weight (P less than 0.001), the regression line intersecting 100 ml/kg at 2,000 gm and 70 ml/kg at 4,500 gm. This correlation has clinical application in the management of polycythemic neonates with partial exchange transfusion. A nomogram is provided for estimating BV/kg in polycythemic neonates based on birth weight.