A group of low-birth-weight infants with daily weight gains that were below the expected mean for postnatal age were examined to determine the effects of RBC transfusion on their weight gain. The mean hemoglobin concentration (+/- SD) in 13 infants (birth weight less than 1,500 g) prior to transfusion was 8.5 +/- 1.6 g/dL and 11.4 +/- 2.1 g/dL after transfusion. When a comparison was made between the daily weight gain for the week prior to transfusion with the week following transfusion, the mean daily weight gain (+/- SD) increased from 20.8 +/- 4.6 g to 28.0 +/- 6.3 g. Among the six infants with pretransfusion hemogloblin concentrations of less than 7.5 g/dL, the increase in daily weight gain was greatest (a rise from 22.6 +/- 4.0 g to 34.1 +/- 4.9 g). Improvements in weight gain were associated with a decrease in metabolic rates as determined by declines in oxygen consumption.