A substantial number of patients with COPD are underweight (UW); they comprise the clinical subtype of "dyspneic" or emphysematous. To determine whether these patients are more dyspneic than normal weight (NW) patients with COPD, we quantitated the severity of dyspnea, using a modified Medical Research Council (MRC) dyspnea scale, in 33 UW and 57 NW patients and compared their pulmonary function tests (PFTs), arterial blood gases (ABGs), and respiratory muscle strength as estimated by maximum static inspiratory (PI(max)) and expiratory (PE(max)) mouth pressures (all as means +/- SEM). Body mass index was 18.7 +/- 1.2 and 24.5 +/- 1.8 kg/m(2) in UW and NW patients, respectively (p < 0.0001). The MRC dyspnea scale was 3. 1 +/- 0.9 in UW and 2.5 +/- 1.2 in NW groups (p = 0.035). All PFT and ABG parameters were similar in the two groups except for DCO (36 +/- 11% in UW and 57 +/- 17% in NW, p < 0.001) and PI(max) (55 +/- 18 mm Hg in UW and 66 +/- 19 mm Hg in NW, p = 0.020). In a stepwise multiple regression model, %DCO and %MVV combined were the best predictors of dyspnea severity (R(2) = 0.30, p = 0.001). We conclude that UW patients with COPD are more dyspneic than NW patients. Although the origin of dyspnea in COPD is multifactorial, changes in DCO and respiratory muscle strength may contribute to its intensity.