BACKGROUND: Consistent with the increasing prevalence of obesity in the United States and many countries worldwide, anesthesiologists are now presented with a greater number of adult and pediatric patients who are significantly overweight. This prospective study was designed to examine the relation between age-adjusted body mass index, preoperative comorbidities, and perioperative outcome in children. METHODS: Children aged 2-18 yr undergoing noncardiac elective procedures were classified as overweight or obese based on their age- and sex-adjusted body mass index. Information was elicited regarding patient demographics, presence of comorbidities, and anesthetic technique. Data regarding the incidence and severity of perioperative adverse events were collected prospectively. RESULTS: Two thousand twenty-five children comprised the sample (1,380 normal weight, 351 overweight, and 294 obese). Obese children had a significantly higher prevalence of comorbidities than nonobese children, including asthma, hypertension, sleep apnea, and type II diabetes. Furthermore, obese children had a higher incidence of difficult mask ventilation, airway obstruction, major oxygen desaturation (>10% of baseline), and overall critical respiratory adverse events. Logistic regression analysis revealed several risk factors for adverse events, including procedures involving the airway, obesity, age younger than 10 yr, and a history of obstructive sleep apnea. CONCLUSIONS: These results suggest that children presenting for elective surgical procedures who are obese have a greater prevalence of preexisting comorbid medical conditions and an increased incidence of perioperative adverse respiratory events compared with normal-weight children. Identification and awareness of risk factors for perioperative complications will be important in optimizing the anesthetic management of these children.