To understand the mechanisms of respiratory system compensation to internal loading during sleep, all-night sleep studies were performed in 10 patients with chronic stable asthma. We used noninvasive measurements to identify the onset of increased airway resistance in sleep. In each sleep study, we recorded arterial oxygen saturation (SaO2) and an array of electromyograms (diaphragm, external intercostal and sternomastoid) as well as thoracoabdominal motion. Only 4 patients developed acute asthma during sleep. A total of 6 such attacks were recorded. The attacks were detected by audible wheeze, augmentation of diaphragm, external intercostal and sternomastoid activity, associated with distinctive changes in thoracoabdominal motion. The duration of these acute asthmatic attacks ranged between 20 and 140 min. One attack started in stage I/II non-rapid-eye-movement (NREM) sleep, 3 in stage III/IV NREM sleep, and 2 in rapid-eye-movement (REM) sleep. Acute asthma in NREM sleep resulted in a paradoxical inward displacement of the abdomen during early inspiration. Attacks occurring during REM sleep resulted in rib cage inward displacement during inspiration. Attacks occurring during REM sleep resulted in rib cage inward displacement during inspiration. Attacks occurring in both NREM and REM sleep did not result in a significant fall in SaO2. We conclude that acute internal respiratory loading during sleep can provoke different compensatory mechanisms in order to provide adequate ventilation in adult asthmatics.