Nasal intermittent positive pressure ventilation (NIPPV) applied during sleep has been demonstrated to be useful in the treatment of restrictive thoracic diseases (RTD). The purpose of this study was to evaluate the repercussions of a withdrawal period from NIPPV of 15 days. This would be sufficient time for patients to go on trips without the respirator. It was hypothesized that once daytime improvement was achieved and was stable, it could be maintained for this period of time. Five volunteer patients with severe RTD who had been receiving treatment with nocturnal NIPPV for at least 2 months before and who had improved at least 5 mm Hg in daytime PO2 and PCO2 were included in the study. No significant differences were disclosed clinically or with arterial blood gas levels, spirometry results, lung volumes, airway resistances, or maximal muscle pressures 15 days following the withdrawal. However, in the sleep studies, a severe worsening of gas exchange was observed, mainly during rapid eye movement (REM) sleep, as well as a trend toward a more disturbed sleep pattern and more important alterations in cardiac rhythm. Consequently, withdrawing the treatment with nocturnal NIPPV cannot be recommended, at least for this particular removal period. Moreover, alterations in daytime gas exchange were found to originate in those produced during REM sleep through the blunting of the respiratory center to CO2. The NIPPV obstructs this mechanism, preventing the deterioration of gas exchange during sleep.