To determine the incidence and duration of hypoxemia in the postanesthesia care unit (PACU), 200 patients were investigated in a single-blind observer study. The number of unrecognized hypoxemic episodes, as well as risk factors and possible association between hypoxemia and postoperative morbidity, were studied. Oxygenation was monitored continuously with a pulse oximeter. One or more hypoxemic episodes (Spo2≤ 90%) were noted in 55% of the patients. Spo2values ≤ 80% were noted in 13% of the patients. Supplementary oxygen was given during 55% of the 447 hypoxemic episodes registered. The hypoxemic episodes were unrecognized by the staff in 95% of the cases. With stepwise multiple logistic regression analyses, risk factors associated with a higher incidence of hypoxemia were: duration of anesthesia (P< 0.0001), age (P< 0.002) and a history of smoking (P< 0.01). Patients who had undergone regional anesthesia had a lower risk of hypoxemia (P< 0.0002). The occurrence of hypoxemia in the PACU could not be correlated to postoperative morbidity. We conclude that hypoxemic episodes in our PACU are common and that the routine use of supplemental oxygen combined with normal clinical surveillance did not prevent hypoxemic episodes.