We studied anionic sites on the glomerular basement membrane in patients with chronic renal transplant rejection having clinical and histological features of transplant glomerulopathy. All patients had significant proteinuria (greater than 1 g/24 h as well as light- and electron-microscopic features very like focal segmental glomerulosclerosis, though no patient had that disease in the native kidney. A significant reduction in the density and absolute number of anionic sites was observed compared with controls and with patients having stable graft function. The findings suggest that the loss of anionic sites may play an important part in the pathogenesis of transplant glomerulopathy.