To characterize an unusual, sex-linked recessive neuromuscular disease, we studied two families with 37 males who had involvement of distal leg and proximal arm muscle groups. Electromyography and muscle biopsy in five subjects showed features of both neuropathy and myopathy. Bradycardia and syncope in 15 involved subjects were associated with early death (before the age of 50 years). Electrocardiograms in 15 others showed a spectrum of atrial abnormalities that ranged from abnormal P waves to permanent atrial paralysis and from first-degree atrioventricular block to complete heart block. No patient exhibited clinical muscle disease without electrocardiographic atrial disease. Dilated, hypertrophied left ventricles with normal indexes of function were found in three cases with permanent atrial paralysis and chronic junctional bradycardia. Cardiomegaly and cardiac failure were not present in the other cases. We conclude that permanent ventricular pacing (instituted four patients) is indicated in many of these patients to prevent serious sequelae.