The effectiveness of early single volume exchange transfusion (ET; 80 ml/kg) was compared with that of early double volume exchange transfusion (160 ml/kg) for treatment of hemolytic disease of the newborn caused by AB0 incompatibility. Twenty full-term infants with AB0 hemolytic disease were randomized into the two treatment groups. The groups were comparable for gestational age, body weight, hemoglobin values, reticulocyte count, maximum serum bilirubin levels, rate of rise of serum bilirubin before ET, antibody titer, and age at time of ET (all p greater than 0.05). The efficacy of treatment was similar in both groups taking into account the mean bilirubin level after ET, post-ET bilirubin, duration of phototherapy following ET, and frequency of second ET (all p greater than 0.05). However, platelet count immediately after ET was lower in the double volume ET group as compared to the single volume ET group (p less than 0.01). Hemoglobin values immediately after ET were higher in the double volume ET group (p less than 0.01). At ten days of life no differences were detectable. The results of this study indicate that the effectiveness of single volume ET for treatment of full-term infants with jaundice due to AB0 incompatibility is at least comparable to that of double exchange ET. Furthermore, the lesser aggressive approach determines less complications such as a decrease of platelet count.