The glomerular ultrastructure was retrospectively reviewed from 45 renal transplant biopsies with the clinical and light microscopic diagnosis of acute rejection (25 cases) and chronic rejection (20 cases). Three grades of morphologic alteration were ultrastructurally defined. In acute rejection, capillary lumenal obliteration with endothelial cell hypertrophy and cellular infiltration were interpreted as the local glomerular expression of the endothelial vascular alterations of acute humoral rejection and were correlated with a poor graft survival. Graft nephrectomy was necessary in 9 of 11 patients with grade III glomerulopathy. In chronic rejection, thickened basement membranes and increased amount of mesangial matrix were considered the result of ischemia. The degree of ultrastructural glomerular alterations in chronic rejection did not correlate with graft survival. Urine protein values were consistently elevated, although poorly correlated with the severity of glomerular alterations. Recurrent glomerulonephritis was not documented in any case. We concluded that the glomerular alteration in acute rejection is a component of acute humoral rejection and that the degree of glomerulopathy in acute rejection is a good predictor of the graft survival.