To examine the association between anemia and postoperative apnea in former preterm infants, 24 former preterm infants of less than 60 weeks postconceptual age undergoing inguinal hernia repair were studied. A hematocrit of at least 25% was required for study participation. General endotracheal inhalational anesthesia, supplemented with neuromuscular blockade and controlled ventilation, was used. No barbiturates or opioids were administered. Respiratory pattern and heart rate were recorded for at least 12 h postoperatively using an impedance pneumograph. Tracings were analyzed for evidence of apnea, periodic breathing, and/or bradycardia by a pulmonologist unaware of the hematologic profile of the infant. Nineteen patients had a hematocrit of 30% or greater (group 1). Their mean (+/- standard deviation [SD]) gestational age was 33.5 +/- 2.7 weeks and postconceptual age 45.5 +/- 4.6 weeks. Five infants had a hematocrit less than 30% (group 2). Their mean gestational age (+/- SD) was 32.4 +/- 3.2 weeks and postconceptual age 43.6 +/- 5.5 weeks. Anemic infants had an 80% incidence of postoperative apnea versus 21% in infants with a normal hematocrit (P less than .03). In the infants who developed postoperative prolonged apnea and/or bradycardia, a prior history of apnea was equally present in both groups (21% in group 1 and 20% in group 2). This study shows that anemia in former preterm infants can be associated with an increased incidence of postoperative apnea.