A peculiar non-hypertrophic myocardiopathy is described which occurred in three and possibly five generations of a single family. Clinical features included systolic murmurs, electrocardiographic abnormalities, and sudden cardiac death with a paucity of symptoms of cardiac dysfunction. Pathological studies in three generations showed a striking similarity of cardiac findings including globular and dilated ventricles, endocardial fibroelastosis, and mitral valve thickening. Myocardium in two showed basophilic degeneration and fibrosis. A retrospective genealogic analysis and a prospective clinical evaluation of living family members suggested an autosomal dominant mode of inheritance with variable penetrance. The cause of this heritable myocardiopathy is presumably a mutant gene; the biochemical defect to which the mutant gene gives rise remains unknown.