OBJECTIVE: To study the outcome of adenotonsillectomy for obstructive sleep apnea (OSA) in children less than 3 years of age. DESIGN AND SETTING: Prospective study at the University of New Mexico Children's Hospital. Children with OSA underwent pre- and postoperative full-night polysomnography (PSG). Scores were compared using a paired t test. A P-value <0.05 was considered significant. RESULTS: The study population included 20 children. Fifteen (75%) were male. The mean age was 2.2 years (range, 1.1 to 3.0). Sixteen (80%) children had medical comorbidities. Over 25% of children had postoperative complications including laryngospasm and marked desaturations. The mean preoperative respiratory distress index (RDI) was 34.1 and the mean postoperative RDI was 12.2 ( P < 0.0001). After surgery, 7 (35%) children had an RDI < 5. Thirteen (65%) had a postoperative RDI ≥ 5 indicating persistent OSA. CONCLUSION AND SIGNIFICANCE: Children under 3 years show significant improvement in RDI after adenotonsillectomy for OSA, but they may develop complications after surgery. Postoperative PSG is recommended for children under 3 years of age to monitor the severity of persistent OSA. EBM rating: B-2. (Otolaryngol Head Neck Surg 2005;132:681-684.)