A polysomnographic study with noninvasive oxyhemoglobin saturation (HbSaO2) monitoring has been conducted in 11 consecutive male nonobese subjects with chronic obstructive pulmonary disease (COPD). The patients with a high arterial CO2 level and markedly reduced functional residual capacity appeared to be more prone to undergo HbSaO2 desaturation during sleep compared to patients with less severe functional and clinical respiratory impairment. The HbSaO2 desaturations were mostly linked to REM sleep and appeared to be concomitant to the inhibition of the tone of the intercostal muscles. Regarding sleep architecture, the patients with less severe COPD and scarce or absent tendency to HbSaO2 desaturation during sleep showed light and fragmented sleep with a marked tendency to arousals and awakenings.