Pulmonary function has been studied extensively in patients with unilateral diaphragmatic paralysis (UDP), but there is scarce information regarding the respiratory function during sleep in this condition. We therefore studied pulmonary function in 12 patients with UDP when awake and when asleep. Diaphragmatic dysfunction was confirmed by the demonstration of low maximal transdiaphragmatic pressures in most of our patients; paradoxical gastric pressure swing was observed in 6 patients. There was a restrictive pattern in pulmonary function tests and resting arterial blood gases were rather well preserved (range SaO2 90-95%). Overnight sleep monitoring showed that the time spent in REM sleep and stage 3 and 4 sleep was reduced. The mean maximum decrease in SaO2 was 15.2 +/- 6.2% and the time with an SaO2 drop of more than 5% of the awake SaO2 was 25.4 +/- 22.8 min. None of our patients was in respiratory failure or had clinical evidence of cor pulmonale. We conclude that UDP leads to significant nocturnal hypoxemia but, in the absence of systemic lung disease, does not lead to chronic respiratory failure and cor pulmonale.