Cardiomyopathy was diagnosed in several members of two families. This familial cardiomyopathy showed symmetrical or asymmetrical hypertrophy of the ventricular walls with or without obstruction to the left ventricular outflow tract. Certain forms were asymptomatic and were revealed by the family history and echocardiography. Myocardial and intercostal muscle biopsy was performed for a biochemical and ultrastructural analysis. Different myocardial features were observed in the two families: a large increase in the glycogen deposits in the one, without clinical signs of a glycogen storage disease, and intracellular deposits of a filamentous protein substance in the other.