Twenty-two cases of membraneous glomerulonephritis (MGN) were identified among 256 bioptically investigated transplanted patients. MGN was defined as de novo in 15 patients and recurrent in three. The type of MGN could not be ascertained with certainty in the other four. Several morphologic features unusual for the idiopathic form of MGN were found. Most cases disclosed focal segmental distribution of subepithelial deposits and showed the contemporaneous presence of different stages of the disease according to Ehrenreich and Churg classification. In addition mild-to-moderate mesangial cell proliferation was found in about one third of de novo MGN biopsies. Endocapillary hypercellularity was observed in 14 specimens and held to be due to an excess of mononuclear blood cells, related to a concomitant episode of rejection. Chronic transplant glomerulopathy was found in 47% of patients with de novo MGN and in 66% of those with recurrent MGN, being more frequently observed in specimens with diffuse distribution of deposits. Repeated biopsies showed progression of the stage and extension of deposits to a large number of capillary loops in four out of six patients. De novo MGN was documented 1 to 54 mo (mean value 20.2 mo) after transplantation, and the recurrence was observed after 12, 15, and 42 mo. All but two patients (who were anuric) complained of proteinuria, which was in the nephrotic range in 12. Apart from the significantly higher frequency in de novo MGN patients of DR4 antigen, whose significance must in any case be re-evaluated in a larger series, none of the factors so far suggested to be linked to the onset of de novo MGN has found further support in our study. On the contrary, relevance in favoring the appearance and the evolution of MGN has to be attributed to transplant glomerulopathy, which, moreover, seems to be more important than MGN itself in causing the unfavorable outcome of the graft.