- Objective We aimed to examine the hypothesis that behavioral and neurocognitive functions of preschool children with Obstructive Sleep Apnea Syndrome (OSAS) are impaired compared to healthy children, and improve after adenotonsillectomy (TA). Methods A comprehensive assessment battery was used to assess cognitive and behavioral functions, and quality of life in children with OSAS compared to matched controls. Results 45 children (mean age 45.5 ± 9 months, 73% boys, BMI 15.7 ± 2) with OSAS were compared to 26 healthy children (mean age 48.6 ± 8 months, 46% boys, BMI 16.4 ± 2). Mean AHI in the OSAS group was 13.2 ± 10.7 (ranging from 1.2 to 57). Significantly impaired planning and fluency (executive function) were found in children with OSAS, as well as impaired attention and receptive vocabulary. Parents and teachers described the OSAS group as having significantly more behavior problems. Quality of life questionnaire in children with OSAS (mean 2.3, range 0.7–4.3) was significantly worse compared to controls (mean 0, range: 0–4), P < 0.004. One year following TA, 23 children with OSAS and 18 controls were re-evaluated. Significant improvement was documented in verbal and motor fluency, sustained attention, and vocabulary. After TA, fewer behavioral problems were seen. Conclusions Preschool children with OSAS present significantly impaired executive functions, impaired attention and receptive vocabulary, and more behavior problems. One year after TA, the prominent improvements were in behavior and quality of life. These findings suggest that the impact of OSAS on behavioral and cognitive functions begins in early childhood. Pediatr Pulmonol. 2012; 47:180–188. © 2011 Wiley Periodicals, Inc.