We tested the efficacy of nocturnal nasal ventilation (NNV) using the BIPAP ventilator in patients with restrictive thoracic diseases by withdrawing them from NNV for an average of 1 wk. One male and five female patients were enrolled in the study; four with restrictive chest wall diseases, and two with muscular dystrophies. All patients had chronic CO2 retention (PaCO2 greater than 50 mm Hg) and had been improved by using NNV for at least 2 months before the study. Four patients were switched to the BIPAP ventilator from standard portable volume ventilators at least 1 month prior to the study without changes in gas exchange or symptoms. After withdrawal of NNV, patients had no deterioration in daytime vital signs, pulmonary functions, maximal inspiratory or expiratory pressures, or arterial blood gases compared with measures made immediately before withdrawal and 1 wk after resumption. However, patients had more dyspnea at rest, increased daytime somnolence, more morning headaches, less daytime energy, and felt less rested in the morning during withdrawal of NNV. Furthermore, nocturnal monitoring demonstrated greater tachycardia, tachypnea, oxygen desaturation, and hypoventilation during withdrawal of NNV. We conclude that NNV administered by the BIPAP ventilator is effective in ameliorating nocturnal hypoventilation and daytime symptoms in patients with chronic CO2 retention caused by severe restrictive thoracic diseases. These data also suggest that the efficacy of NNV may depend more on amelioration of nocturnal hypoventilation than on resting of ventilatory muscles.