21diabetic patients were examined neurologically and neurophysiologically and were divided into two groups: one with neuropathy (10 patients) and the other without neuropathy (11 patients). Ultrastructural dimensions and density of capillaries in the gastrocnemius muscle were examined in 11 patients. Capillary basement membranes were significantly thicker in patients with neuropathy (p < 0.05), whereas there was no difference in endothelial, luminal or pericytial fractions of the total capillary area. No difference was found in capillary densities or capillary/fiber (C/F) ratios. Muscle blood flow was estimated using the 133Xe clearance method in all the patients, and was found to be reduced in the 4 patients with the most severe neuropathy (p < 0.01) even though there was no difference on the whole between the groups with or without neuropathy. The results suggest that the development of diabetic neuropathy is associated with basement membrane thickening. The causes of the basement membrane thickening are discussed.