Oxygen saturation determined by pulse oximetry was monitored in 152 pediatric surgical patients divided into two groups. In one group, the oximeter data and alarms were available (N=76) to the anesthesia team, and, in the other group, these data were unavailable (N=76). A trained observer recorded all intraoperative hypoxic episodes and informed the anesthesia team of all major events (i.e., oxygen saturation ≤ 85% for ≥30 s) (Pao2 approximately 52 mm Hg). Thiry-five major events occurred: 24 in the unavaiable group, and 11 in the available group (P = 0.021). A greater number of major events occurred in children ≤ 2 yr of age (P = 0.013). Hypoxic events diagnosed by the oximeter, but not by the anesthesiologist, were more frequent in the unavailable group (13) than in the available group (5) (P = 0.0495). ASA Physical Status 3 and 4 patients were more likely to suffer a major event (P = 0.009 available, 0.006 unavailable). The pulse oximeter diagnosed hypoxmia before the signs and symptoms of hypoxemia were apparent (i.e., prior to observed cyanosis or bradycardia). Major hypoxic events were unrealated to duration of anesthesia. Major events were evenly distributed among induction, maintenance, and awakening from anesthesia; a greater number of hypoxic events occurred during induction in the unavailable group (P = 0.031). No morbidity was documented in any patient who suffered an hypoxic event. More patients experienced borderline oxygenation in room air at the end of anesthesia (≥90% saturation) in the unavailable group (12 of 60) than in the available group (3 of 57) (P = 0.009). The authors conclude that pulse oximetry, in contrast to changes in vital signs, does provide and early warning of developing hypoxemia in anesthetized children.