Small children are physiologically subject to arterial oxygen desaturation. However, few reports have referred to the risk factors related to postanesthetic hypoxemia and the duration of hypoxemia. The purpose of this study was to clarify these two aspects. Eighty-five ASA physical status I infants and children were included in the study. They were scheduled for minor surgery. Fifty-six underwent oral endotracheal intubation, and 29 patients breathed from a mask. Anesthesia was maintained with Enflurane or Halothane and nitrous oxide. Arterial oxygen saturation was measured with a pulse oximeter. The measurements were started shortly after patients' arrival in the recovery room, and conducted every 5 min at least for 1 hour. Ten patients had SpO2 values of less than 95%. In all except one, SpO2 decreased within 10 min after arrival in the recovery room. Age, height, and weight of these 10 children were significantly different from the remaining 75, but there were no significant differences in anesthetic duration and postanesthetic awakefulness between the group with postanesthetic hypoxemia and the one without. The importance of monitoring the clinical condition of pediatric patients after general anesthesia is universally acknowledged. Monitoring with the pulse oximeter has proven very useful and shows that, unless oxygen saturation is monitored, all children should receive supplemental oxygen.