The muscle capillaries of diabetic subjects, with and without retinopathy, have been measured. The groups with retinopathy had significantly thicker laminae than those without the fundal changes. No significant difference was found between the group with proliferative retinopathy and the group with nonproliferative retinopathy. The focal and segmental nature of the basal lamina thickening was confirmed by the increasing standard deviation of the measurements within and among capillaries. This study also confirms the fact that, at least for muscle capillaries, an apparent relationship exists between the thickness of the basal lamina of these vessels and the presence of clinical retinopathy.