Childhood obesity has become a global epidemic. It is related to several chronic diseases such as essential hypertension, type 2 diabetes mellitus, and renal disease. The relationship between the degree of obesity and lung functions is well defined in adults, but limited information is available about the childhood period. This study aims to determine the impact of the degree of obesity on the pulmonary functions of school children and adolescents. Cross sectional study. Included in the study were a total of 170 school children and adolescents (9-17 years old) referred to our paediatric outpatient clinic. Of these subjects, 42 were lean and non-obese (BMI % <85), 30 subjects were overweight (BMI % >85, <95), 34 subjects were obese (BMI % >95, <97), and 64 subjects were morbidly obese (BMI % >97). Anthropometric measurements were taken and spirometry was performed on all subjects. Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), forced vital capacity 25-75 (FEV25-75) and peak expiratory flow (PEF) were used to measure the ventilatory functions for all the subjects. The groups showed no significant differences in age or gender. Despite no statistically significant differences in FEV1, FVC, or FEV1/FVC, there were significant reductions in PEF (p<0.001) and FEV25-75 (p<0.001) in the overweight, obese and morbidly obese subjects, when compared with those who were non-obese. Overweight, obese and morbidly obese children have no obstructive abnormalities compared with healthy lean subjects.