We have used three-dimensional reconstructions obtained with spiral computed tomography to measure total diaphragm length (Ldl) and surface area (Adl), the length (Ldo) and surface area (Ado) of the dome, and the length (Lap) and surface area (Aap) of the zone of apposition in 10 hyperinflated patients with severe chronic obstructive pulmonary disease, or COPD (FEV1 = 27% predicted: FRC = 225% predicted) and 10 normal subjects matched for age, sex, and height. Measures of Ldl, Adl, Lap, and Aap decreased linearly between FRC and TLC in the two groups, but Ldo and Ado did not change. On average, patients' Adl and Aap at FRC were reduced to 73% and 54% of normal values, whereas Ado was unaffected. When compared at similar absolute lung volumes, mean diaphragm dimensions were similar in patients with COPD and normal subjects, but individual values were very variable in both groups. This variability was partly accounted for by differences in body weight: i.e., the greater the weight, the longer the diaphragm. We conclude that (1) patients with COPD have marked reductions in Adl and Aap at FRC but have diaphragm dimensions similar to those of normal subjects when compared at similar absolute lung volumes, and (2) normal subjects and patients with COPD show substantial intersubject variability in diaphragm dimensions that is partly explained by differences in body weight.