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A woman with widespread myokymia presented initially at age 19 with hoarseness and mild dyspnea on exertion. The diagnosis of Isaacs syndrome was suggested by the clinical findings of widespread continuous muscle activity and depressed tendon reflexes, although she lacked the usually prominent increased muscle tone. The diagnosis was confirmed by the electromyographic demonstration of continuous spontaneous muscle action potentials that were abolished by neuromuscular blockade but not by local nerve blockade. Pulmonary function tests were consistent with fixed extrathoracic obstruction. The vocal cords were closely approximated. Electromyographic studies of the laryngeal muscles under general anesthesia revealed continuous muscle activity, which accounted for the hoarseness and much of the exertional dyspnea. The patient responded well to treatment with phenytoin and carbamazepine. This unusual syndrome should be considered in the differential diagnosis of patients with respiratory complaints and muscle fasciculations, even though they have normal muscle tone.