A study was conducted during the first week of life to determine the changes in P50 (PO2 required to achieve a saturation of 50% at pH 7.4 and 37 degrees C) and the proportions of fetal hemoglobin (HbF) and adult hemoglobin (HbA) prior to and after transfusion in very early preterm infants. Eleven infants with a gestational age < or = 27 weeks have been included in study. The hemoglobin dissociation curve and the P50 was determined by Hemox-analyser. Liquid chromatography was also performed to determine the proportions of HbF and HbA. The mean gestational age of the 11 infants was 25.1 weeks (+/- 1 weeks) and their mean birth weight was 736 g (+/- 125 g). They received 26.9 mL/kg of packed red cells. The mean P50 prior and after transfusion was 18.5 +/- 0.8 and 21.0 +/- 1 mm Hg (P = .0003) while the mean percentage of HbF was 92.9 +/- 1.1 and 42.6 +/- 5.7%, respectively. The data of this study show a decrease of hemoglobin oxygen affinity as a result of blood transfusion in very early preterm infants prone to O2 toxicity. The shift in HbO2 curve after transfusion should be taken into consideration when oxygen therapy is being regulated for these infants.