Patients with myotonic dystrophy classified clinically into function groups were studied. Muscle strength for knee extension and flexion was, with few exceptions, moderately or markedly reduced with successively more severe clinical disability. The reduction in maximal walking speed showed a similar tendency. There was a large percentage of both type I and in relation to normal findings of type II C fibers in biopsies from most patients. The fiber area varied greatly even in the patients in the best function groups, some of whom had large type II fibers. The most common-histopathological changes, found in all function groups, were fiber atrophy with small angular fibers, internal nuclei, splitting, fibrosis and moth-eaten fibers. A high percentage of type I fibers can already be seen in patients with practically no other morphological changes and without significant functional deterioration.