The study was designed to show whether there was any relation between muscle capillary basement membrane thickness, HLA-antigens, anti-insulin antibodies and proliferative retinopathy. Electron microscopic measurements of muscle capillary basement membrane thickness were performed on muscle biopsies from 15 insulin-dependent diabetics and severe proliferative retinopathy, 24 insulin-dependent diabetics with minimal retinopathy and 18 age- and sex matched non-diabetics. All the patients had had diabetes for 20 years or more. None had biochemical or clinical evidence of diabetic nephropathy. Basement membrane thickness was measured according to the methods of Siperstein and Williamson. Muscle capillary basement membrane thickening occurred in 32 of 39 diabetics, using the Siperstein method, but patients with proliferative retinopathy did not exhibit thicker basement membranes than patients with no or minimal changes in the retina. There were apparent differences in HLA-antigens between diabetics with and without proliferative retinopathy, but they did not reach statistical significance. There was no correlation between muscle capillary basement membrane thickness and the quantity of insulin antibodies. The results indicate that factors other than basement membrane thickening and genetic factors in the HLA-region, are responsible for the development of proliferative retinopathy.