A substantial number of patients with chronic obstructive pulmonary disease (COPD) suffer from gradual and significant weight loss during the natural course of their illness. The aim of this study was to determine the contribution of resting energy expenditure (REE) and energy intake (E-Intake) to weight loss in 80 patients with stable COPD: age (mean +/- SEM) 65 +/- 3 yr and FEV1 1.0 +/- 0.1 L. A total of 39 patients exhibited a continued weight loss in the previous year. REE measured with a ventilated hood system was significantly (p less than 0.005) higher in weight-losing compared to weight-stable patients when values were normalized for predicted metabolic rate (117 +/- 3 and 108 +/- 2%, respectively) or kg fat-free mass using bioelectrical impedance (FFM-BI): 35.0 +/- 0.8 and 31.8 +/- 0.6 kcal/kg, respectively). Normalized values of E-Intake were not significantly different between weight-losing and weight-stable patients. REE/FFM-BI correlated significantly with FEV1 (r = 0.22, p less than 0.05), maximal inspiratory mouth pressure (Plmax; r = 0.35, p less than 0.001), and E-Intake/FFM-BI (r = 0.48, p less than 0.001). Classification in three subgroups by severity of disease, (1) FEV1 greater than 35%, (2) FEV1 less than or equal to 35%, and (3) PaO2 less than 7.3 kPa, revealed a higher prevalence of weight loss in the more compromised groups and a significantly decreased (p less than 0.05) E-Intake in the hypoxemic patients.(ABSTRACT TRUNCATED AT 250 WORDS)