Morbid obesity, the most significant risk factor for development of sev- eral respiratory diseases, is linked to decreased pulmonary function. The aim of this study was to determine the relationships between pulmonary function and plasma levels of homeostasis model assessment-insulin resistance (HOMA-IR), insulin, leptin, hs-CRP and fasting glucose. Values were measured in 39 Thai children and adolescents, divided into three groups according to lung function (forced expiratory volume in one second, FEV1); normal (Group A) FEV1 ≥ 80% (n = 19), obese normal (Group B) FEV1 ≥ 80% (n = 14) and obese (Group C) FEV1 < 80% (n = 6). Body mass index was highest in group C. Groups A and B were comparable for FEV1, forced vital capacity (FVC), maximal voluntary ventilation (MVV) and FEV1/FVC, whereas Group C exhibited significantly reduced FEV1, FVC and MVV but a normal FEV1/FVC ratio. All values except the FEV1/FVC ratio were significantly lower than in groups A and B. Group C had significantly higher levels of leptin, insulin, FG and HOMA-IR than Groups A and B (p < 0.001). There was a significant negative correlation between FEV1 and MVV with leptin, insulin and HOMA-IR, but not with high-sensitivity C-reactive protein (hs-CRP). We conclude that FEV1 is reduced in obese children and adolescents and inversely correlates with plasma leptin, insulin and HOMA-IR levels. We have shown that the most important factor in inducing a restrictive lung in these patients may be related to leptin status.