The effect of sleep state on the ventilatory pattern and chest wall mechanics was studied in 8 asthmatic adolescents (mean age +/- SD, 13.6 +/- 1.3 yr) using a respiratory inductive plethysmograph and surface electromyogram (EMG) electrodes. During rapid eye movement (REM) sleep we observed a mean decrease of 43% in intercostal EMG activity. This was associated with paradoxical inward motion of the rib cage to the extent that the rib cage compartment contributed negatively to tidal volume. Accompanying the abnormal chest wall mechanics there was a mean increase of 45 +/- 38% in diaphragmatic EMG activity, as well as a substantial increase in abdominal contribution to tidal volume. However, despite the increased diaphragmatic activity, tidal volume decreased during REM sleep. The abnormal chest wall mechanics during REM sleep were also associated decreased mean inspiratory flow rates and prolongation of the duty cycle (TI/Ttot). The mean maximal decrease in hemoglobin oxygen saturation during sleep compared with that during wakefulness was 3.9%.