Thirteen children with abnormal mitochondria in muscle tissue, and a progressive neurological disorder that affected the cerebrum, cerebellum, extrapyramidal system, vestibular system, retina, upper motor neuron, lower motor neuron, and musculature, are reported. Other signs and symptoms were short stature, diabetes mellitus, cardiopathy, hypoplastic anaemia, glomerulopathy, and renal tubular dysfunction. These symptoms may occur singly or in various combinations and the manifestation may differ even within the same family. The most common clinical picture was that of "ophthalmoplegia plus'. Occurrence in relatives varied from isolated symptoms to the complete syndrome with "ragged red fibres' and is not inconsistent with an autosomal dominant mode of inheritance with variable expressivity. Theories for the pathophysiological basis of this syndrome are discussed and the literature reviewed.