We studied the effects of oral nutritional supplementation on respiratory muscle (RM) performance in 25 ambulatory patients with severe chronic obstructive pulmonary disease (COPD). There was a relationship between body weight and anthropometric parameters of nutritional status (triceps skinfold thickness [r = 0.67; p less than 0.005], midarm muscle circumference (r = 0.53; p less than 0.005), but body weight did not correlate with daily caloric intake, serum albumin, transferrin, or blood lymphocyte count. None of these measurements of nutritional status correlated with any measure of RM strength or endurance. In a randomized observer-blinded crossover trial, patients were allocated to one of two groups. In the first eight weeks of the study, group A received nutritional supplementation, and patients in group B were control subjects. In the second eight weeks, patients in group A were control subjects, and group B received supplement. Mean daily caloric intake and body weight increased in both groups while receiving supplement (both p less than 0.05). Calories provided by the supplement were frequently substituted for normal dietary calories. Any increases in RM performance in the group receiving supplement were matched by increases (due to learning) in controls. We conclude that oral dietary supplements have no important effects on RM performance in ambulatory patients with COPD.