OBJECTIVES: It remains uncertain whether treatment with adenotonsillectomy for obstructive sleep apnea in children improves cognitive function. The Preschool Obstructive Sleep Apnea Tonsillectomy and Adenoidectomy study was a prospective randomized controlled study in which researchers evaluated outcomes 12 months after adenotonsillectomy compared with no surgery in preschool children symptomatic for obstructive sleep apnea.METHODS: A total of 190 children (age 3-5 years) were randomly assigned to early adenotonsillectomy (within 2 months) or to routine wait lists (12-month wait, no adenotonsillectomy [NoAT]). Baseline and 12-month assessments included cognitive and behavioral testing, medical assessment, polysomnography, and audiology. The primary outcome was global IQ at 12-month follow-up, measured by the Woodcock Johnson III Brief Intellectual Ability (BIA). Questionnaires included the Pediatric Sleep Questionnaire, Parent Rating Scale of the Behavioral Assessment System for Children-II, and Behavior Rating Inventory of Executive Function, Preschool Version.RESULTS: A total of 141 children (75.8%) attended baseline and 12-month assessments, and BIA was obtained at baseline and 12-month follow-up for 61 and 60 participants in the adenotonsillectomy versus NoAT groups, respectively. No cognitive gain was found after adenotonsillectomy compared with NoAT, adjusted for baseline; BIA scores at 12-month follow-up were as follows: adenotonsillectomy, 465.46 (17.9) versus NoAT, 463.12 (16.6) (mean [SD]). Improvements were seen for polysomnogram arousals and apnea indices and for parent reports of symptoms (Pediatric Sleep Questionnaire), behavior (Behavior Assessment System for Children behavioral symptoms, P = .04), overall health, and daytime napping. CONCLUSIONS: Structured testing showed no treatment-attributable improvement in cognitive functioning of preschool children 12 months after adenotonsillectomy compared with NoAT. Improvements were seen after adenotonsillectomy in sleep and behavior by using polysomnogram monitoring and parental questionnaires.WHAT'S KNOWN ON THIS SUBJECT: Studies to evaluate changes in neurocognition after adenotonsillectomy for obstructive sleep apnea suggest that younger children may have potential for greater improvement. The previous study with a randomized design was in school-aged children.WHAT THIS STUDY ADDS: This randomized study of preschool children showed reduced frequency of day naps and confirmed polysomnographic and behavioral improvements after adenotonsillectomy compared with those still awaiting surgery, but our primary analysis showed no improvement in global IQ.