Intermittent positive pressure ventilation reduces inspiratory muscle electromyographic activity among patients with restrictive ventilatory failure. It has therefore been suggested that the reduction of energy expenditure at night could result in improved inspiratory muscle function during the day. Reported successes with nocturnal ventilation have not included measurements of inspiratory muscle endurance. We therefore electively ventilated six (five female, one male) patients (mean +/- SD) aged 36 +/- 13 years in whom respiratory failure (room air PaCO2, 60 +/- 13 mm Hg; PaO2, 44 +/- 11 mm Hg; SaO2, 75 +/- 12 percent) was consequent on restrictive ventilatory disease (vital capacity, 25 +/- 7 percent predicted; FEV1/FVC, 81 +/- 12 percent; total lung capacity, 40 +/- 5 percent predicted; MIPRV -42 +/- 10 cm H2O; MEP, 81 +/- 28 cm H2O). Positive pressure ventilation was administered with a customized closely fitting nasal mask attached to a volume-cycled pressure-limited ventilator. Full respiratory polysomnographic measurements as well as arterial blood gases, pulmonary function, distance walked in six minutes, and inspiratory muscle endurance were measured at baseline and after 3 and 14 months of ventilation. Ventilation improved saturation (baseline on O2; SWS 87 +/- 10, REM 79 +/- 14, ventilator on R/A; SWS 90 +/- 6, REM 89 +/- 5 percent) and transcutaneous Pco2 (baseline on O2; SWS 85 +/- 26, REM 94 +/- 39, ventilator on R/A; SWS 53 +/- 9, REM 58 +/- 9 mm Hg). During ventilation, the quantity and distribution of sleep was similar to that observed prior to ventilation. Daytime gas exchange improved as did the six-minute walking test (initial test = 429 +/- 120 m, three months after ventilation = 567 +/- 121 m), both of these improvements being sustained at 14 months. Inspiratory muscle endurance measured using a pressure threshold load (mean mouth pressure = 45 percent MIPRV) improved from 7.1 +/- 3.4 minutes at baseline to 14.8 +/- 7.6 minutes at 3 months, an improvement sustained at 14 months. There was no change in measured lung volumes or respiratory muscle strength. We conclude that the improvement in nocturnal gas exchange, daytime functioning, and arterial blood gases resulting from nocturnal positive pressure ventilation is associated with an increase in inspiratory muscle endurance sustained at 14 months.