In chronic congestive heart failure (CHF), backward effects of left ventricular dysfunction alter pulmonary volumes and gas diffusion. Some of these disorders are detected in some patients with diabetes mellitus, possibly due to a microangiopathic process and nonenzymatic glycosylation of lung tissue proteins. We explored the possibility that coexistence of non-insulin-dependent diabetes mellitus (NIDDM) may potentiate the deterioration of lung function in CHF. In 20 normoglycemic patients (group 1) and in 20 patients with NIDDM (group 2), with New York Heart Association class II to III CHF due to idiopathic or ischemic cardiac disease, and in 20 controls (groups were age- and gender-matched), we investigated cardiac function, pulmonary volumes, carbon monoxide diffusion (DL_CO) and its alveolar-capillary membrane (D_M) subcomponent, oxygen uptake and dead space-to-tidal volume ratio (pVD/VT) at peak exercise (individualized ramp test), and slope of ventilation-to-carbon dioxide production ratio (VE/VCO₂) during exercise. Although, compared with reference subjects, both patient groups had similar variations in left ventricular diastolic volume, ejection fraction, and pulmonary wedge pressure; in group 2 lung volumes, DL_CO, D_M, and oxygen uptake were significantly more reduced; in this group there was no overlap of individual results of DL_CO and D_M with those in controls; VE/VCO₂ slope and pVD/VT also were significantly increased, and inversely correlated with D_M. Thus, coexistence of NIDDM makes pulmonary dysfunction worse in CHF, and significantly enhances exercise intolerance.