Nocturnal nasal intermittent positive pressure ventilation (NIPPV) is an effective means of normalizing awake blood gases in patients with respiratory insufficiency due to neuromuscular or chest wall dysfunction. However, little attention has been paid to the effects of long-term ventilation on spontaneous breathing during sleep in such patients. The purpose of this study was to determine whether spontaneous breathing during sleep improved after long-term nasal ventilation. Fourteen patients with documented nocturnal respiratory failure, who had been treated with nocturnal NIPPV for at least 6 months, were reviewed with an all night polysomnograph on a night without ventilatory support. The severity of nocturnal desaturation both in non-rapid eye movement (NREM) and rapid eye movement (REM) sleep without nocturnal ventilation was compared to desaturation measured during the initial diagnostic study. Spontaneous daytime blood gas values (mean +/- SD) were significantly improved at follow-up compared to values obtained prior to nasal ventilation: arterial oxygen tension (Pa,O2): 7.5 +/- 1.2 to 10.2 +/- 1.3 kPa (p < 0.005); arterial carbon dioxide tension (Pa,CO2) 8.2 +/- 1.6 to 6.4 +/- 0.7 kPa (p < 0.001). Significant improvements in inspiratory muscle strength were also observed with maximal inspiratory pressure (Pl, max) increasing from a baseline value of 41 +/- 18 to 65 +/- 26% predicted measured prior to the night of ventilation withdrawal (p < 0.003). Spontaneous breathing during sleep after long-term treatment was markedly improved although still abnormal. During NREM sleep without ventilatory support, oxygen desaturation was significantly less severe compared to the initial study (arterial oxygen saturation (Sa,O2) 88 +/- 4 vs 78 +/- 8%; p < 0.001). Minimum Sa,O2 during REM sleep similarly improved from a mean value of 49 +/- 14% during the diagnostic night to 73 +/- 10% at review follow-up (p < 0.001). In 12 patients, transcutaneous carbon dioxide was measured continuously during sleep on both occasions and demonstrated significantly less CO2 retention during follow-up compared to control studies both in NREM (p < 0.003) and REM sleep states (p < 0.004). Long-term nocturnal ventilation produces improved respiratory drive both asleep and awake and improved arousal responses to abnormal blood gases.