The purpose of this study was to determine the incidence and severity of obstructive events and oxyhemoglobin desaturation (dSaO2) in 37 patients with paralytic/restrictive ventilatory insufficiency during use of nocturnal ventilatory assistance provided by means of negative-pressure body ventilators (BVs). Thirteen of the 37 patients had mean oxyhemoglobin saturation (SaO2) less than 95 percent and a mean of ten or more episodes per hour when the dSaO2 was greater than or equal to 4 percent (4%dSaO2/h). In all, 26 of the 37 patients had evidence of significant multiple episodes of dSaO2 while asleep on BVs. Polysomnography performed on three of these patients substantiated the obstructive nature of the dSaO2. Twenty-two of the 37 patients who had a mean SaO2 of 90.6 +/- 7.2 percent and a mean of 17.7 +/- 16.1 4%dSaO2/h on BVs were switched to noninvasive ventilatory assistance by intermittent positive airway pressure (NV-PAP). Their mean SaO2 improved to 96.0 +/- 2.2 percent, and the 4%dSaO2/h decreased to 1.2 +/- 1.8 per hour. All symptoms similar to those of obstructive sleep apnea were relieved. We conclude that BV use is associated with significant dSaO2 in over 50 percent of patients. The dSaO2 is predominantly obstructive in nature but may be due to chronic underventilation in patients using less effective BVs. Patients with a mean SaO2 less than 95 percent or 10 or more 4%dSaO2/h may benefit from conversion to NV-PAP via the nose, the mouth, or an oral-nasal interface.