Four patients with mitochondrial abnormality had multiple muscle biopsies at several year intervals during which respiratory chain enzyme activities were shown to be quite variable. In three patients, progression of the disease paralleled the decrease in respiratory chain enzyme activity. In one patient, the clinical and pathological findings improved with age as is seen in the benign infantile form of cytochrome c oxidase (CCO) deficiency. The variability in these mitochondrial disorders may result from the varied proportions of normal and abnormal mitochondria in the muscle cells in which the mitochondria are said to be randomly replicated from numerous mitochondrial DNA copies.