Nocturnal oxygen desaturation during the sleep is very frequent in patients affected by chronic obstructive pulmonary disease (COPD). Hypoventilation, rather than sleeping apnea, is commonly considered as the most relevant factor in the onset of nocturnal oxygen desaturation. On this topic, the Authors have carried on a study on the nocturnal hypoxemia in 70 hospitalized COPD patients with a mean FEV1% of 40 +/- 21 and a mean PaO2 of 67.7 +/- 6.1. Anthropometric features (sex, age, body mass index) and functional respiratory parameters (FEV1, FEV1/VC, PaO2, PaCO2, SaO2, pH) were considered. Moreover all the patients were monitorized with transcutaneous pulse oxymetry, while breathing environmental air, in nighttime. Mean oxyhemoglobinic nocturnal saturation (SaO2 noct.%), minimum registered value of nocturnal SaO2 (min SaO2 noct.%) and the minutes of nighttime SaO2 < or = 90% and < or = 85% (tSaO2 < or = 90% e < or = 85%) were considered. Fiftyfour patients (77.15%) were nocturnal desaturating (NOD), whereas 16 (22.85%) were not desaturating (nNOD). A statistically significant difference was found between the two groups as to the values of FEV1 (p < 0.05), PaCO2, pH, SaO2 noct.%, minimum SaO2 noct.% and tSaO2 < or = 90% and < or = 85% (p < 0.0001). A statistically significant correlation was found between tSaO2 < 90% and BMI (r = 0.44), PaCO2 (r = 0.48) and pH (r = -0.44), as well as between tSaO2 < 85% and PaCO2 (r = 0.57) and pH (r= -0.50), between SaO2 noct.% and BMI (r = -0.45), PaCO2 (r = -0,50), FEV1 (r = 0.44) and pH (r = 0.46) and finally between minimum SaO2 noct.% and PaCO2 (r = -0.47) was found. Eighty percent of the NOD patients had PaO2 < 75 mm Hg and PaCO2 > 44 mm Hg. All the patients with PaCO2 > 50 mm Hg were NOD. In conclusion, all COPD subjects with FEV1 < 49% and daytime PaO2 > 60 mm Hg, particularly when associated to elevated PaCO2 values and high BMI, should undergo a nocturnal pulse oxymetry in order to identify possible nocturnal desaturations. In these patients reduced FEV1, high BMI and/or elevated PaCO2 appear to be predictive indexes of nocturnal desaturation. A PaCO2 > 50 mm Hg is highly indicative for a nocturnal oxygen desaturation.