BACKGROUND Membranous glomerulopathy is a common complication of renal allograft. However, its incidence and prognosis are not well defined, because an undetermined number of them pass undiagnosed under the generic epigraph of chronic allograft nephropathy. METHODS To assess the diagnostic refinement supplied by electron microscopy to conventional light and immunofluorescence procedures the authors reviewed 17 cases of electron microscopy-confirmed membranous glomerulonephritis in kidney allograft. In addition, they searched for other features of graft injury, particularly lesions associated with alloimmune reaction, in order to evaluate the contribution of each lesion to the long-term outcome of the allograft. RESULTS In 4 of the 17 cases of their series the diagnosis of membranous glomerulopathy was made by electron microscopy. In addition, in 5 samples, lesions of chronic alloimmune rejection were present (in 4 cases the diagnosis was based on electron microscopy findings). At the end point of the study, 3 of the 5 patients with chronic alloimmune injury were in dialysis, 1 had died with functioning allograft, and the fifth suffered severe renal failure but was not in dialysis. On the other hand, 3 of the 12 patients without evidence of alloimmune injury had returned to the dialysis program. CONCLUSIONS Electron microscopy is a useful tool in the assessment of renal allograft pathology and can provide additional morphological features of prognostic relevance.