We report the case of a patient with myotonic dystrophy who developed tachypnea and severe dyspnea without respiratory failure. Myotonia of inspiratory muscles was diagnosed on the grounds of marked prolongation of transdiaphragmatic pressure (Pdi) decay during sniffs. In view of the recognized sensory role of inspiratory muscles in dyspnea, it was hypothesized that antimyotonic therapy might relieve dyspnea in this patient. Procainamide therapy induced a decrease in half relaxation time of Pdi during sniffs and yielded a striking clinical improvement with cessation of tachypnea and dyspnea. Later, this beneficial effect was maintained by tocainide after procainamide was stopped because of a lupus syndrome. We conclude that myotonia of respiratory muscles can cause severe dyspnea that can be improved by antimyotonic therapy.