Six European treatment centers contributed to a controlled trial to study nocturnal hypoxemia in COPD patients having daytime PaO2 of 60-70 mmHg. The trial is composed of two parts: first, patients inclusion, taking men aged under 70 years, excluding sleep apnea syndrome and all other concomitant pathologies leading to nocturnal desaturation. We described this population and determined the frequency and degree of nocturnal desaturation. Correlations between different daytime and sleep parameters have also been established. The second part concerns the study of the two sub-populations of nocturnal desaturators and nondesaturators. This is followed by random allocation of the desaturators to oxygen or no oxygen treatment. We defined significant nocturnal desaturation as cumulated unsaturation period, exceeding 30% of total time, in bed spent under SaO2 lower than 90%. Twenty out of 46 patients were desaturators (43%). Only the first part is dealt with in this article.