Pulmonary mechanics and respiratory muscle pressures were studied in 7 patients with severe respiratory muscle weakness. Minimal pleural pressures were grossly abnormal and showed little variation with lung volume. Both the maximal transpulmonary pressure and static expiratory compliance were low; therefore, the pressure-volume curves of the lungs resembled those obtained after strapping the chest in normal subjects. The low compliance may result from either microatelectasis or a generalized alteration in alveolar elastic properties and is probably a major determinant of both the total lung capacity and the breathing pattern of patients with neuromuscular disease. Airway and gas exchange function were less abnormal than the elastic properties of the lungs.