Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction during sleep. Inspiratory muscles may be subjected to potentially fatiguing loads during an obstructive apnea and this may be related to the termination of obstructive apnea. We have measured transdiaphragmatic pressure (Pdi) and breathing patterns in six male patients with OSA during sleep to characterize respiratory muscle function in OSA and determine whether apnea termination is consistently related to a pressure time index of the diaphragm (PTI) associated with respiratory muscle fatigue. There was a large intersubject variability in Pdi generation during apnea. No consistent level of PTI was associated with apnea termination. During prolonged apneas, the respiratory duty cycle plateaued, which is suggestive of an inhibitory reflex possibly mediated by chest wall afferents. There were intersubject differences in both inspiratory and expiratory muscle recruitment during apnea. In the majority of patients, the diaphragm appeared to be the primary inspiratory muscle during apnea, but in some it appeared to be the intercostal/accessory muscles. The majority of patients demonstrated an increase in gastric pressure and inward abdominal movement during the expiratory phases of an apnea, consistent with abdominal muscle recruitment stimulated by increased ventilatory drive.