Background: Overweight and obese children are at increased risk of a wide range of health conditions including respiratory diseases. In addition, inactivity can decrease pulmonary function. This study assessed the effect of obesity and inactivity on pulmonary function impairment in adolescents. Materials and Methods: This study was conducted on 80 adolescents. Subjects were divided into two groups. Group I included 40 untrained (VO2max= 29.30±4.20) fat adolescents (UO). Group II included 40 healthy trained (VO2max= 58.11±2.23) normal weight adolescents (TN). Body mass index (BMI), body fat percentage and waist to hip ratio (WHR) were calculated and pulmonary function tests were carried out according to the standard protocols. Data were analyzed using student’s “t” test and Pearson’s correlation coefficient. Results: UO had significantly lower pulmonary function values than the TN group. They also showed lower FEV1/FVC ratio when compared to TN group (p <0.05). In UO group, BMI, body fat percentage and WHR had a significant negative correlation with pulmonary function whereas in TN group only BMI had significant negative correlation with pulmonary function. A significant decrease in FEV1 was observed in the two groups, which led to a decrease in FEV1/FVC% after the exercise compared to before. Thus, exercise test induced airway resistance in both groups. Conclusion: untrained obese adolescents have more respiratory symptoms than their normal weight trained peers, and these factors are recommended to be used as a predictor of pulmonary function in assessment of obese children in epidemiological studies. In addition, obesity and inactivity can surcharge pulmonary function abnormalities in adolescents.