1. The concentration of metabolites in intercostal and quadriceps muscle, and pulmonary function, were studied in twelve patients with chronic obstructive lung disease and acute respiratory failure before, during and after standardized treatment at an intensive care unit. The findings were compared with those obtained in hospitalized patients of comparable age with non-pulmonary diseases. 2. On admission, when the patients had marked hypoxaemia, hypercapnia and acidosis, the concentrations of ATP and creatine phosphate were low in both intercostal and quadriceps muscle, particularly the latter. The lactate concentration was increased in relation to control values but glycogen did not differ significantly. 3. In response to therapy, the Pa,CO2 and the patient's acidosis decreased, the vital capacity increased and lung mechanics improved along with the clinical condition. At the same time there were significant increases in the concentrations of ATP, creatine phosphate and glycogen in intercostal and quadriceps muscles, to values similar to, and for glycogen in excess of, those found in control subjects. Lactate concentration fell significantly during treatment. 4. In view of the low initial muscle concentrations of ATP and creatine phosphate in the patients, it is suggested that dysfunction of the respiratory muscles may be an important component of respiratory failure. Moreover, the concentration of energy-rich compounds in muscle rose significantly as the patients responded to treatment, which emphasizes the importance of adequate nutritional therapy in this disorder.