We conducted a retrospective chart review to examine the efficacy of adenotonsillectomy for the treatment of severe obstructive sleep apnea (OSA) in children. Our study population was made up of 85 patients-58 boys and 27 girls, aged 1 to 17 years (mean: 6.9 ± 4.4)-with severe OSA who had undergone adenotonsillectomy and pre- and postoperative attended polysomnography (PSG) over a 4-year period. Severe OSA was defined as an apnea-hypopnea index (AHI) of >10 events per hour of sleep. Patients who had an underlying genetic or craniofacial anomaly were excluded. In addition to demographic and PSG data, we compiled information on selected characteristics of patients according to postoperative residual AHIs of ≤5 and >5. Finally, information on body mass index z score was available on 72 patients; the mean score was 1.55 ± 1.51, with 36 patients (50.0%) fulfilling the criteria for obesity. In the group as a whole, we found that adenotonsillectomy resulted in a significant reduction in AHI from 35.4 to 7.1 (p < 0.001). We also found an improvement in mean oxygen saturation nadir from 75.2 to 85.5 (p < 0.001). Postoperatively, only 8 patients (9.4%) achieved an AHI of ≤1; AHIs were >1 to ≤5 in 39 patients (45.9%), >5 to ≤10 in 24 patients (28.2%), and >10 in 14 patients (16.5%). A significantly higher proportion of boys had a residual AHI of >5 after surgery compared with those whose postoperative AHI was ≤5 (78.9 vs. 59.6%; p = 0.04). We conclude that adenotonsillectomy leads to a significant improvement in sleep-disordered breathing in children with severe OSA, but residual disease is common so close postoperative follow-up is essential.