Home oxygen therapy is now considered indicated in cases of severe sleep oxygen desaturation. To explore whether we could choose cases with severe sleep desaturation by measurement of waking physiological parameters, we examined the relationship between arterial blood gases, lung functions and pulmonary artery pressure while awake and the degree of sleep oxygen desaturation in 21 patients with chronic obstructive pulmonary disease and 22 cases with a past history of pulmonary tuberculosis. There were significant correlations between waking PaO2, PaCO2 and pulmonary artery mean pressure and sleep lowest SaO2, respectively, whereas no significant relation was found between %VC or FEV1.0% and sleep lowest SaO2. However, we found wide ranges of PaO2 and PaCO2 in cases who had 10 min or more with SaO2 below 85% during sleep. This was also true in the patients who underwent 60 min or more with SaO2 below 85% while a sleep. These results suggest that it may be difficult to find out the indications for cases with severe sleep desaturation by evaluation of arterial blood gases while awake.