The patient described in this report was an 11-year-old girl with a negligible heart murmur. The length was below 10th percentile. There appeared to be a cardiomyopathy with, at that moment, no signs of obstruction. There were no evident clinical symptoms of myopathy of skeletal muscle. However, electromyography was myopathic. Biochemical studies revealed no abnormalities. In muscle biopsy sections, the Sudan staining reactions revealed the presence of large amounts of sudanophilic droplets, predominantly in type I fibers. Electronmicroscopy demonstrated lipid excess and accumulations of enlarged, mostly rounded mitochondria in a subsarcolemmal location, with closely packed cristae. In many mitochondria a dense osmiophilic material was present in the spaces between the cristal membranes. The patient's parents were first cousins. One of her brothers who died at the age of 4 1/2 years presumably suffered from the same disease. The pattern of inheritance is most probably autosomal recessive.