To study the relationship between anemia and breathing abnormalities in preterm infants, we measured oxygen supply and demand in two groups of anemic infants less than or equal to 32 weeks of gestational age. Six-second apnea density was less than 1% in one group and greater than or equal to 5% in the other. There were no differences in hemoglobin concentration, available oxygen, oxygen consumption, or Doppler-determined cardiac output between the two groups of infants. Furthermore, in anemic preterm infants with apnea density greater than or equal to 5%, reductions in 6-second apnea density were similar after erythrocyte transfusion (mean +/- SEM: from 8.6% +/- 1.1% to 4.7% +/- 0.7%) or after an isovolemic infusion of 5% albumin (from 9.0% +/- 1.4% to 4.7% +/- 0.7%). These results show no relationship between measures of oxygen delivery and respiratory irregularities, and indicate that volume expansion may play a role in ameliorating the pneumocardiogram abnormalities.