Patients with myotonic dystrophy often develop respiratory failure caused by alveolar hypoventilation. Abnormalities in the ventilatory response to hypoxia and hypercapnia may explain this phenomenon. Accordingly, hypoxic and hypercapnic responses were measured in seven patients with myotonic dystrophy who had only mild respiratory muscle weakness. Hypoxic response was significantly reduced, while hypercapnic response was affected more irregularly. It is possible that the high incidence of respiratory failure in such patients is related to decreased hypoxic ventilatory response, occurring because of an underlying neurogenic deficit.