The aim of this study was to investigate if results of overnight pulse-oximetry (OPO) predict survival in COPD patients undergoing long-term therapy (LTOT). 97 COPD patients (74 M and 23 F) qualified for LTOT from 1986 to 1992 were studied. When qualifying for LTOT patients were mean age 59 years old, had severe airway obstruction (mean FEV10,9L), hypoxaemia (mean PaO2 53 mmHg) and hypercapnia (mean PaCO2 48 mmHg). Two OPO were performed in each patient: the first on atmospheric air and the second while breathing oxygen. All patients were observed for at least 2 years or until death. Fifty five patients died during the observation time. All deaths were related to the primary disease. Survival analysis by Cox formula showed the prognostic value of mean, initial and minimal arterial oxygen saturation while breathing air and of initial oxygen saturation while breathing oxygen. Also classical factors, such as age or FEV1, had the prognostic value in this group of patients. CONCLUSIONS results of overnight oxygen saturation affect prognosis in COPD patients despite long-term domiciliary oxygen treatment.