In two siblings with limb-girdle muscle weakness and episodic headaches and vomiting from early childhood, progressive neurologic degeneration later developed, and both children died. In one child, corticosteroids induced improvement in both cerebral and muscular symptoms that lasted 1 year. This patient had elevated blood, urine, and spinal fluid lactate levels, together with increased cardiac output and oxygen consumption at rest. Several muscle fibers were characterized by a "ragged red" appearance with the trichrome stain. Subsarcolemmal and intermyofibrillar excess of mitochondrial oxidative enzyme reaction product was correlated with abnormal mitochondrial aggregates by electron microscopy. The brain revealed focal areas of cortical degeneration and necrosis with adjacent gliosis or edema. Ferrocalcific deposits were prominent in the globus pallidus. The other sibling had similar changes in the brain at autopsy. This familial multisystem disorder especially involving the brain, skeletal muscle, and heart appears to represent a defect in some mitochondrial oxidative mechanism.