Background: The aim of this study was to determine the frequency and type of respiratory complications after urgent adenotonsillectomy (study group) for comparison with a control group of children undergoing a sleep study and adenotonsillectomy for obstructive sleep apnea syndrome. A second aim was to assess risk factors predictive of respiratory complications after urgent adenotonsillectomy. Methods: The perioperative course of children who underwent adenotonsillectomy between January 1, 1999, and March 31, 2001, was reviewed. Two groups of children were identified from two different databases: the hospital database for surgical procedures (the study group) and the sleep laboratory database (the control group). The retrospective chart review focused on the preoperative status (including an evaluation for obstructive sleep apnea), anesthetic management, and need for postoperative respiratory interventions. Results: A total of 64 consecutive cases for urgent adenotonsillectomy were identified, and 54 children met the inclusion criteria. Thirty-three children (60%) had postoperative respiratory complications necessitating a medical intervention; 11 (20.3%) required a major intervention (reintubation, ventilation, and/or administration of racemic epinephrine or Ventolin), and 22 (40.7%) required a minor intervention (oxygen administration). Six children (11.1%) required reintubation in the recovery room for respiratory compromise. Risk factors for respiratory complications were an associated medical condition (odds ratio, 8.15; 95% confidence interval, 1.81-36.73) and a preoperative saturation nadir less than 80% (odds ratio, 5. 54; 95% confidence interval, 1.15-26.72). Sixteen (49%) of the medical interventions were required within the first postoperative hour. Atropine administration, at induction, decreased the risk of postoperative respiratory complications (odds ratio, 0.18; 95% confidence interval, 0.11-1.050. Control group: Of 75 children who underwent a sleep study and adenotonsillectomy, 44 had sleep apnea and were admitted to hospital after elective adenotonsillectomy. Sixteen (36.4%) children had postoperative respiratory complications necessitating a medical intervention. Six percent of the children (n = 3) required a major medical intervention. No child required reintubation for respiratory compromise. Conclusions: Severe obstructive sleep apnea syndrome and an associated medical condition are risk factors for postadenotonsillectomy respiratory complications. Risk reductions strategies should focus on their assessment.