Although it is known that in patients with COPD acute hyperinflation determines shortening of the inspiratory muscles, its effects on both diaphragm and rib cage morphology are still to be investigated. In this preliminary study the relationships between hyperinflation, emphysema, diaphragm and rib cage geometry were studied in 5 severe COPD patients and 5 healthy subjects. An automatic software was developed to obtain the 3-D reconstruction of diaphragm and rib cage from CT scans taken at total lung capacity (TLC) and residual volume (RV). Dome surface area (Ado), radius of curvature, length (Ld) and position (referred to xiphoid level) of the diaphragm and antero-posterior (A-P) and transverse (T) diameters of rib cage were calculated at both volumes. Ado and Ld were similar in COPD and controls when compared at similar absolute lung volumes. Radius of curvature was significantly higher in COPD than in controls only at TLC. In COPD, the range of diaphragm position was invariantly below the xiphoid level, while in controls the top of diaphragm dome was always above it. Rib cage diameters were not different at TLC. A-P diameter was greater in COPD than in controls at RV, while T diameters were similar. In conclusion, in severe COPD diaphragm and rib cage geometry is altered at RV. The lower position of diaphragm is associated to smaller A-P but not transversal rib cage diameters, such that rib cage adopts a more circular shape.