Biopsy specimens from the gastrocnemius or rectus femoris muscle of 20 patients with intermittent claudication were studied using fresh frozen cryostat sections and histochemical reactions for adenosine triphosphatase, nicotinamide adenine nucleotide dehydrogenase reductase and phosphorylase and modified Gomori trichrome staining. Neuropathic changes, such as fibertype grouping and small group atrophy, were present to some extent in all of the biopsy specimens. Myogenic muscle changes such as necrosis and phagocytosis were seen in approximately one third and various forms of myofibrillar disorganization in approximately two thirds of the specimens. The amount and size of the type I aerobic fibers increased with the increasing severity of the ischemic disease.