We studied the ventilatory pattern and mouth occlusion pressure (P0.1) at rest and the response to awake hypercapnic stimulation in 27 patients with obstructive sleep apnea syndrome (OSAS). The respiratory pattern was characterized by both increased P0.1 and VT/Ti (p < 0.05), with a higher VE (p < 0.01) due to a slight increase in VT. Ventilatory response to hypercapnic stimulation showed no significant differences with respect to the control group. Our findings reveal that OSAS patients show an increased minute output secondary to a mild degree of hyperstimulation on the baseline central ventilatory control and that there is a subgroup of patients with a decreased ventilatory response to CO2.