OBJECTIVE : To investigate the prevalence of obstructive sleep apnea (OSA) in children referred for obesity treatment, and to compare the prevalence with that of a normal-weight group. Moreover, we examined the association between Body Mass Index Standard Deviation Score (BMI SDS) and the Apnea-Hypopnea Index (AHI). METHODS : This cross-sectional study included 139 children aged 7-18 years with overweight/obesity (BMI SDS >1.28) recruited from an obesity treatment clinic. The normal-weight group consisted of 33 children (BMI SDS ≤ 1.28) aged 7-18 years recruited from schools. Sleep examinations were performed using a type 3 portable sleep monitor (Nox T3). OSA was defined as AHI ≥ 2. Height and weight were measured and the tonsillar size was clinically estimated using the Brodsky scale. RESULTS : The OSA prevalence was 44.6% in children with overweight/obesity compared with 9.1% in the normal-weight group (p = 0.0002), and the relative risk of OSA was 4.9 (95% CI 1.6-14.7). In a logistic regression, a one-unit increase in the BMI SDS increased the odds of having OSA by a factor of 1.92 independent of age, sex, tonsillar hypertrophy, and asthma (95% CI 1.33-2.76, p = 0.0005). A generalized linear regression adjusted for the same variables revealed an association between BMI SDS and AHI (a one-unit increase in the BMI SDS equaled an average increase in the AHI of 35% (95% CI 19-53%, p < 0.0001)). CONCLUSIONS : In this study, children with overweight/obesity had a significantly higher prevalence of OSA compared with a normal-weight group. Increased BMI SDS was associated with increased AHI.