Randomized, blinded. A total of 1,231 patients in the metropolitan Montreal area were screened. Of these, 348 were recruited to enter a four week stabilization period, and 184 were subsequently randomized to either active or sham negative pressure ventilation (NPV). A five day in-hospital period was used to train patients in ventilator use and obtain baseline measures of exercise capacity, lung function, respiratory symptoms, and quality of life. Randomization was stratified on the basis of whether or not the patient had been receiving oxygen at home. Home ventilation treatment took place during a following 12-week period. Respirator use was recorded from patient logs and from concealed meters installed in the units. Patients received four home visits by physiotherapists during the 12-week period and returned to the hospital for follow-up at four and twelve weeks post-discharge for reassessment. The principal outcome measure was the distance in meters walked in six minutes. Secondary outcome measures were cycle exercise endurance time, severity of dyspnea, quality of life, arterial blood gas tensions, and respiratory muscle strength.