In seven cases of idiopathic progressive ophthalmoplegia with abnormal biopsies of clinically normal or only minimally weak somatic muscles, histochemical and ultrastructural examination showed the muscle fiber abnormality to consist of clusters of normal and abnormal mitochondria and excessive lipid droplets ("ragged-red" fiber). That change was considered myopathic. The occasional small angular fibers, excessively dark with reduced diphosphopyridine nucleotide-tetrazolium reductase resembling those present in denervation, suggested an additional minor element of neuropathy in these cases. The described morphologic abnormalities, in combination with idiopathic progressive ophthalmoplegia, may represent a distinct clinicopathologic syndrome, "oculocraniosomatic neuromuscular disease with ragged-red fibers." Additional features usually include elevated cerebrospinal fluid protein levels and sometimes cardiac conduction abnormalities, retinal pigmentation, impaired growth, and sensorineural hearing defect, abnormal electroencephalogram, and ataxia.