Marked reductions in pulmonary compliance have been noted in patients with respiratory muscle weakness, which greatly contribute to the losses of lung volume. In an attempt to reverse this abnormality and to decrease the work of breathing, we evaluated the effects of a 15-min period of positive pressure, mechanical hyperinflation of the lungs in 10 patients with generalized neuromuscular disorders and long-standing involvement of the respiratory muscles. The vital capacity was 46, and the static expiratory lung compliance was 59% of control values. The recoil pressure of the lung at full inflation was 19 cm H2O. Using positive pressure breathing devices, we inflated the lungs either with inflation pressures ranging between 20 and 40 cm H2O or with volume that were at least twofold larger than the patients' natural inspiratory capacity. Lung volumes, including functional residual capacity, and static pulmonary compliance were not modified by these procedures. Maintaining 10 cm H2O of positive end-expiratory pressure did not alter the results. These findings suggest that (1) alveolar collapse, rather than a simple decrease in the compliance of the surface film of the lung, is the major cause of the low pulmonary compliance in patients with chronic respiratory muscle weakness; (2) these patients do not acutely benefit from intermittent positive breathing treatment.