It is unknown whether daytime features predict oxygenation during sleep in COPD patients with normoxaemia or mild hypoxaemia. In this study our purpose was to evaluate by a pulse oxymeter, nocturnal desaturation in 33 COPD with PaO2 > 60 mmHg and to examine some daytime parameters as possible predictors of nocturnal hypoxaemia. A significant nocturnal desaturation has been defined by spending > or = 30% of total sleep-time with a TSTSaO2 < 90% > 30. According to this criterion we classified our patients in Desaturators (D) and Non Desaturators (ND). Our results showed that 39% of our patients were D and 61% ND. Among anthropometric and respiratory functional data we found that daytime SaO2B (r = 0.74 p < 0.001) daytime PaO2 (r = 0.47 p < 0.01) and daytime PaCO2 (r = 0.45 p < 0.05) were significantly correlated with the nocturnal oxygen desaturation and can predict the presence of sleep related hypoxaemia. In conclusion, our study confirms that a relatively high percentage of COPD patients with normoxaemia or borderline hypoxaemia exhibits significant nocturnal hypoxaemia. Further studies will suggest whether sleep related hypoxaemia deserves nocturnal oxygen therapy.