OBJECTIVE: To assess the respiratory function and sleep characteristics of obese adults and children.METHODS: All patients with non-syndromic, severe obesity (BMI ≥3 z-scores for children and ≥40.00kg/m2RESULTS: A total of 69 children (mean±SD BMI 36.8±6.7 and mean BMI z-score 4.7±1.0) and 70 adults were included (mean BMI 45.7±6.2). Metabolic syndrome was diagnosed in 13 children (26%) and 40 adults (80%). Reduced lung volumes were observed in 34 children (50.0%) and 16 adults (24.0%) and both the mean functional residual capacity (FRC) and the mean residual volume (RV) were lower in children than in adults (FRC: -1.7±2.1 z-score in children vs. -1.0±1.1 in adults, P=0.026; and RV: -0.8±1.2 z-score in children vs. -0.1±1.1 in adults, P=0.002). The prevalence of severe obstructive sleep apnea syndrome was greater in adults (40.7% vs. 18.8%, P=0.007). Children had a higher average oxygen saturation (median of 96.0% [91.0-98.0] vs. 93.0% [76.0-97.0] in adults, P<0.0001).CONCLUSIONS: Obesity has consequences for lung volumes in children; however, a longitudinal study is needed to determine the impact on pulmonary expansion and growth.