An eight-year-old boy with slowly progressive muscle weakness was found to have Oil red O positive vacuoles in predominanty type i muscle fibers. Subsequent studies demonstrated markedly reduced skeletal muscle carnitine (0.24 mumoles per gram; normal 1.64 to 3.34). Serum carnitine was normal. Although both parents were clinically normal, muscle carnitine levels were low in both (mother 0.60; father 0.90 mumoles). There was no clinical evidence of cardiac disease but the patient had ventricular hypertrophy by electrocardiography, vectorcardiography, and echocardiography. Treatment with prednisone resulted in clinical improvement but no change in muscle histology. Our studies suggest that the carnitine deficiency of muscle in this case may be due to impaired carnitine entry into muscle and that this form of disease can be inherited as an autosomal recessive disorder.