This report describes a patient with primary alveolar hypoventilation who, after 2 yr of successful treatment with nocturnal oxygen, developed severe hypoxemia and hypercapnia during sleep, morning headaches, and daytime fatigue. Sleep studies demonstrated prolonged periods of hypoventilation and apnea without evidence of upper airway occlusion. Therefore, a phrenic nerve stimulator was implanted to allow pacing of the diaphragm during sleep. However, diaphragm pacing was accompanied by paradoxical movement of the rib cage and upper airway occlusion during sleep, and was unsuccessful in maintaining adequate ventilation. Therefore, the patient underwent a tracheostomy after which diaphragm pacing maintained adequate nocturnal ventilation; however, paradoxical movement of the rib cage persisted. The induction of upper airway occlusion as a result of diaphragm pacing, in contrast to the absence of occlusion during spontaneous breathing, highlights the importance of the normal temporal coordination of inspiratory activation of the upper airway muscles and diaphragm. The findings have important implications for the pathogenesis of obstructive sleep apneas in general.