Abstract : Obesity may affect the respiratory functions in many ways. The adipose tissue deposited around the rib cage, abdomen and visceral cavity affects work of breathing and reduces the pulmonary functions. There are different tests to measure lung function or pulmonary function known as pulmonary function tests. To determine the relation between body mass index and lung functions in obese and overweight children with Down syndrome. A cross-sectional study design was directed to 30 normal children and 60 children with Down syndrome (30 with obesity and 30 with overweight range), aged 7 - 9 years. Body mass index and lung function (spirometry) were evaluated. There was no significant differences between groups concerning the reference values of lung functions. Moreover, there was a negative correlation between BMI and lung functions in the chosen age group children with Down syndrome. Overweight and obese children with Down syndrome demonstrated similar values of lung volume and capacity as normal children. Thus, anthropometric measures and body composition characteristics may not be prognostic features for changed lung function