Patients with myasthenia gravis (MG) may develop heart disease. Our data on 108 MG patients were examined to assess the type and frequency of this. 17 of 108 patients (16%) showed signs of heart disease which could be regarded as MG-related. 11 of these, of whom 5 died suddenly, had clinical symptoms, mainly arrhythmias. Signs of heart disease were most frequent in thymoma patients (5 of 10), and all 3 microscopically examined hearts from these patients showed a focal myocarditis. 44 spinal muscular atrophy patients serving as controls showed a 16% frequency of signs of heart disease of unknown etiology. However, only 1 of 44 spinal muscular atrophy patients (2%) had clinical symptoms as compared to 11 of 108 MG patients (10%). Together with the characteristic focal nature of the myocarditis and the microscopic similarities between lesions of heart and skeletal muscle, this indicates that the heart disease is specifically related to MG.