Ventilatory responses to progressive hypercapnia were analyzed in the normocapnic and hypercapnic obstructive sleep apnea patients (OSA). The rebreathing hypercapnic and hypoxic tests were performed using the computerized equipment (Lungtest, MES), according to Read's method. The ventilatory response to hypoxia was impaired in all OSA patients. Concerning the hypercapnic ventilatory response, there were no differences between the OSA patients with normal end-tidal PCO(2) and controls. Nine moderately hypercapnic OSA patients showed a right shift with a normal slope of the regression curve describing the relationship between the end-tidal PCO(2) and minute ventilation. In contrast, three severely hypercapnic OSA patients showed a right shift with a decreased slope of this regression curve. We conclude that awake OSA patients who developed hypercapnic ventilatory insufficiency showed an impaired hypercapnic defense reaction.