Gastrocnemius muscle biopsies from 19 patients with chronic renal failure were studied by electron microscopy. Thirteen of the biopsies contained pathological muscle fibers. Myopathic findings, such as autolysis and phagocytosis, were seen in eight. Neurogenic findings, such as small atrophic fibers, were observed in seven. Non-specific changes, such as myofibrillar degeneration, abnormal vacuoles, glycogen accumulation, and lipofuscin pigment were also frequent findings. The structural changes from normal were quantitatively more frequent and qualitatively more severe in the muscles of the hemodialysis patients in comparison with those seen in the other patient groups (diet, transplant).