Bilaterally obtained muscle biopsies (anterior tibial muscle) from 22 patients with unilateral peripheral arterial insufficiency (intermittent claudication) were qualitatively and quantitatively analyzed by means of enzyme histochemistry and electron microscopy. In a majority of the biopsies from the symptomatic legs, two different patterns of morphological deviations were seen in the abnormal muscles: one suggested primary muscle fiber damage and the other indicated denervation. Biopsies from the asymptomatic legs were often only slightly changed or normal. Significant correlations were found in both legs between the relative number of type 1 (slow twitch, highly oxidative) fibers and the age of the subjects. In symptomatic legs, furthermore, a relation could be demonstrated between the relative number of type 1 fibers and (1) the initial walking tolerance (walking distance covered before pain occurred), (2) duration of symptoms, and (3) occlusion or stenosis at high level (i.e., above the deep femoral artery). Finally, the diameters of both type 1 and type 2 fibers in the symptomatic leg were correlated to the initial walking tolerance. The importance of circulatory disturbances as differential diagnostic points (arteriovasculo-occlusive myopathy) in the diagnosis of neuromuscular diseases is stressed.