Hypoventilation contributes to oxyhemoglobin desaturation during rapid-eye-movement (REM) sleep in patients with severe chronic obstructive pulmonary disease (COPD). Due to hyperinflated lungs these patients have mechanically impaired diaphragms and increased activity of other inspiratory muscles while awake. We speculated that rib cage (RC) inspiratory muscles might lose activity during REM, thereby contributing to hypoventilation. We therefore recorded scalene (SCA) and sternocleidomastoid (SCM) electromyorgrams in six subjects with severe COPD. SCA activity decreased 76% (P less than 0.001), from non-REM (NREM) to tonic REM and decreased an additional 17% during phasic REM. SCM activity was much more variable during NREM but when present also decreased during REM. SCA activity correlated strongly with RC excursion. SCA and SCM activity, RC excursion, estimates of minute ventilation, and oxyhemoglobin saturation all decreased in parallel. Expiratory activity of the SCA and SCM, present during wakefulness and NREM, disappeared during REM. We conclude that loss of inspiratory activity of rib cage muscles during REM causes chest wall distortion and hypoventilation in patients with severe COPD. Loss of expiratory activity of these muscles may contribute to a decrease in end-expiratory volume and thereby to a deterioration of pulmonary gas exchange.