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Preterm and ex-preterm infants are at risk for life-threatening apnea after general anesthesia. The authors attempted to define the postconceptual age beyond which apnea is less likely to occur and to identify the factors that predispose to postanesthetic apnea.Ninety-one infants younger than 60 weeks postconceptual age undergoing 101 general anesthetics were prospectively studied. All infants were admitted to the hospital for cardiorespiratory monitoring overnight. The presence of coexistent medical conditions that could influence the incidence of apnea was determined by a review of current and previous medical records and by history.Of 38 procedures performed in 35 infants younger than 44 weeks postconceptual age, 10 (26.3%) procedures in 9 infants were associated with postanesthetic apnea. In eight of these, apnea did not resolve spontaneously: four infants required stimulation, and four required continuous positive airway pressure by face mask. Apnea occurred after 2 of 63 procedures performed in infants 44 weeks postconceptual age or older. Both episodes occurred in one patient who had neurologic disease. Of the remaining 61 procedures in the latter age group, 7 were associated with episodes of bradycardia (lowest heart rate was 79 beats/min) postoperatlvely without apnea. These episodes lasted up to 5 s, were not associated with apnea or cyanosis, and resolved spontaneously in all cases. All infants with postanesthetic apnea and/or bradycardia experienced their first episode within 12 h after surgery.Ex-preterm infants younger than 44 weeks postconceptual age are at greater risk for apnea after general anesthesia than are infants older than 44 weeks postconceptual age. Based on these results, the maximum long-run risk of postanesthetic apnea in preterm infants older than 44 weeks postconceptual age is 5% with 95% confidence.