A family (mother and two sons) have had lifelong muscle weakness and intolerance to fatty food. Histochemistry of muscle biopsies of all three patients demonstrated increased lipids in type I muscle fibers and type II muscle fiber atrophy and paucity. Electronmicroscopy of muscle revealed increased lipids, abnormal mitochondria, and increased lipofuscin granules. Electronmicroscopy of sural nerve showed inclusions in most of the Schwann cell cytoplasm, with lipid droplets, zebra bodies, lipofuscin granules, and abnormal mitochondria. Carnitine and CPT I and II levels were normal in serum and muscle. Treatment with long-chain fatty-acid-free diet resulted in remarkable clinical improvement and in decrease of lipid droplets in the muscle. This dietary program may be useful in other forms of lipid myopathy.