Red blood cell volume was estimated indirectly from plasma volume and venous hematocrit measurements in 262 consecutively delivered premature infants of less than 37 weeks' gestation. Infants with respiratory distress syndrome averaged lower red cell volumes (P0.02) than those without. Fatal cases had the lowest volumes. There was a 10.3% mortality from RDS (respiratory distress syndrome) among the one third of infants with the smallest red cell volumes, and only a 2.3% mortality among the one third with the largest volumes. Red cell volume was shown to be a direct correlate of time of cord clamping and thereby of the amount of placental transfusion, both in infants with and without RDS. From this study it is probable that delayed cord clamping, by allowing placental transfusion, decreases the risk of death from RDS in premature infants. Delay in clamping the umbilical cord for 1-1.5 min is advised in premature births.