Negative pressure ventilation (NPV) was applied for 6 to 8 h/day for 2 consecutive days in 13 patients with severe airflow limitation and chronic respiratory failure. After cessation of NPV, the mean arterial blood gases were improved in 10 patients, and this improvement was sustained for the nex 2 days in eight patients, for 3 days in seven patients, and was still present in four patients on the fourth day. Respiratory muscle strength improved in all patients, but there was no relationship between the increase in strength and sustained improvement in gas exchange. Ventilation and respiratory pattern were unchanged in all patients, but the mean VD/VT fell and VA rose while the VO2 and VCO2 fell. The ventilatory responses to hypoxia and hypercapnia increased in patients who demonstrated sustained improvement in blood gases. The mechanism underlying the sustained improvement in gas exchange following NPV is not clear but is likely multifactorial.