This is a report of a 45-yr-old male patient who developed central sleep apnea syndrome because of hypothyroidism. In response to L-thyroxine therapy, the patient became euthyroid, and the apneic phenomenon disappeared. Previous reports have suggested that hypothyroidism can produce obstructive sleep apnea from either narrowing of the upper airway secondary to deposition of mucopolysaccharides and protein extravasation into the tissues or from abnormalities in ventilatory control. The present patient did demonstrate evidence of profound dysfunction of his respiratory control center: before therapy, the patient manifested blunted ventilatory and occlusion pressure responses to hypoxia and normal responsiveness to hypercapnia; after therapy, hypoxic responsiveness was restored and the ventilatory response to hypercapnia doubled. Hypothyroidism should be included in the differential diagnosis of central sleep apnea.