Five members of the same family, along three generations, presented with hypertrophic cardiomyopathy. Neurological examination, muscle strength, electromyography, and serum creatine kinase were normal. Skeletal muscle biopsy showed abnormal lipid accumulation and carnitine deficiency. In three patients the cardiac symptoms and echocardiographic findings improved after treatment with L-carnitine, 3-4 g daily, and a long-chain fatty-acid-free diet.