|| We investigated the incidence of desaturation during general anesthesia in preoperatively hypoxic (< 92%), and nonhypoxic (≥ 92%) pediatric (n=1,090) and adult (n=5,138) patients. We plotted the patients’ SpO2 value time-courses and assessed desaturation in 6,228 patients. The crude overall incidence (95%CI) for desaturation was 11.1% (9.4-13.1) in the pediatric patients and 0.9% (0.6-1.2) in the adults. The crude incidence of desaturation in the hypoxic pediatric patients was 2.5 times the risk in the nonhypoxic patients: risk ratio (RR) 2.5 (1.8-3.5), p<0.001. The risk of desaturation in the hypoxic adult patients was 20.1 times the risk in the nonhypoxic adult patients: RR 20.1 (10.3-39.2), p<0.001. When the patients were separately stratified by American Society of Anesthesiologists Physical Status (ASA-PS) and by age, the directly adjusted risk-ratio (RRS) showed that the hypoxic pediatric patients had 1.8 and 1.6 times the risk in the nonhypoxic pediatric patients: ASA-PS adjusted RRS 1.6 (1.8-2.2), p<0.001; age-adjusted RRS 1.8 (1.3-2.5), p<0.001, and the hypoxic adult patients had 13.8 times the risk in the nonhypoxic adult patients: RRS 13.8 (6.9-27.6), p<0.001. A pulse-oximeter check before the start of general anesthesia could ensure timely preparation to avoid intraoperative desaturation.