Background: Obesity is a common co-morbidity of COPD and has been associated with worse outcomes. However, it is unknown whether the interaction between obesity and COPD modulates diaphragm shape and consequently its function. The body mass index has been used as a correlate of obesity. We tested the hypothesis that the shape of the diaphragm muscle and size of the ring of its insertion in Non-COPD and COPD subjects are modulated by BMI. Methods: We recruited 48 COPD patients with post-bronchiodilator FEV1/FVC < 0.7 and 29 age-matched smoker/ex-smoker control (Non-COPD) subjects, who underwent chest CT at lung volumes ranging from FRC to TLC. We then computed maximum principle diaphragm curvature in the midcostal region of the left hemidiaphragm at end of inspiration during quiet breathing (EI) and at total lung capacity (TLC). Results: The radius of maximum curvature of diaphragm muscle increased with BMI in both COPD and Non-COPD subjects. The size of diaphragm ring of insertion on the chest wall also increased significantly with increasing BMI. Surprisingly, COPD severity did not appear to cause significant alteration in diaphragm shape except in normal weight subjects at TLC. Conclusion: Our data uncovered important factors such as BMI, the size of the diaphragm ring of insertion, and disease severity that modulate the structure of the ventilatory pump in Non-COPD and COPD subjects.