To find out the predictors of nocturnal arterial oxygen desaturation in patients with respiratory diseases, transcutaneous oxygen saturation(StcO₂) monitoring studies using a pulse oximeter were performed during sleep in 20 patients. StcO₂ was decreased more than 4% from the baseline value in 18 patients(90%) and more than 10%('Desaturator') in 8(40%). Five of the seven patients(71.4%) with awake PaO₂<60mmHg and three of the thirteen patients(23.1%) with awake PaO₂≥60mmHg were 'desaturators'. The awake PaO₂/FIO₂ and PaO₂/PAO₂ could distinguish 'desaturator' from 'nondesaturator, and PaO₂, SaO₂ or StcO₂ could not. These results suggest that the nocturnal oxygen desaturation depends on the severity of the underlying disease rather than the baseline PaO₂. Anthropomorphic and lung function factors could not separate between 'desaturator' and 'non-desaturator', and about a quarter of patients with a wake PaO₂≥60mmHg developed significant desaturation. Therefore, it is necessary to monitor the nocturnal arterial oxygen saturation in patients with respiratory diseases regardless of their severity of airflow obstruction or awake PaO₂.