Reduced pulmonary diffusion capacity is a hallmark of COPD, although the relative contribution of the subcomponents of pulmonary diffusion--membranous component (Dm) and capillary volume (Vc)--is unknown. These components can be measured with the method of NO single-breath diffusion (DLNO). In a prospective study, pulmonary function tests including spirometry, body plethysmography and single-breath measurements of diffusion capacity with CO and NO were performed in 183 patients with COPD of varying severity. There was a severity-dependent decrease in DLCO. Furthermore, Dm as well as Vc was positively correlated with the severity of COPD measured by FEV1. In all stages of COPD, reduction of Vc was more pronounced than constriction of Dm. In patients with most severe COPD, the preponderance of the reduction of Vc was significantly more marked than in milder stages.We conclude that Dm as well as Vc contributes to the reduction of DLCO in COPD, with a predominance of Vc at all stages of COPD. This confirms the idea that the loss of pulmonary capillaries in COPD is functionally relevant.