Fifty-three patients with chronic obstructive airways disease and 19 age-matched controls were studied using computed tomography (CT). The study shows that CT can detect the presence and distribution of pulmonary emphysema. Pulmonary vascular changes detectable on chest radiography correlate well with lung density as measured by CT. Patients with marked CT changes of emphysema had significantly greater impairment of diffusion capacity and FEV1.0/VC than the patients with less severe changes.