To investigate whether a compromised nutritional state may limit exercise performance in patients with chronic obstructive pulmonary disease we studied 54 such patients (FEV1 less than 50% and arterial oxygen tension (PaO2) greater than 7.3 kPa) whose clinical condition was stable and who were admitted to a pulmonary rehabilitation centre. Fat free mass was assessed anthropometrically (from skinfold measurements at four sites) and by bioelectrical impedance; creatinine height index and arm muscle circumference were also assessed. The mean (SD) distance walked in 12 minutes was 845 (178) m. No association was established between the distance walked and spirometric measures. A good correlation was found between the distance walked and fat free mass in the whole group (r = 0.73 for impedance measurements and 0.65 for skinfold thickness) and in a subgroup of 23 lean patients (body weight less than 90% of ideal weight; r = 0.66 for impedance measurements and 0.46 for skinfold thickness). Body weight correlated with the distance walked only in the whole group (r = 0.61). On stepwise regression analysis fat free mass measured by bioelectrical impedance, maximal inspiratory mouth pressure, and PaO2 accounted for 60% of the variation in the distance walked in 12 minutes. We conclude that fat free mass, independently of airflow obstruction, is an important determinant of exercise performance in patients with severe chronic obstructive pulmonary disease.