We studied the ventilatory pattern and mouth occlusion pressure (P-0.1) 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 P-0.1 and V-T/T-i (p < 0.05), with a higher VE (p < 0.01) due to a slight increase in V-T. 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.