Besides rejection-induced transplant glomerulopathy de novo membranous glomerulonephritis (MGN) is the most frequent cause of nephrotic syndrome after renal transplantation. We evaluated 1029 renal transplantations (271 without and 758 with cyclosporine treatment), performed on 848 patients between 1970 and 1992, which resulted in 872 functioning grafts. De novo MGN was seen in 30 biopsy specimens from 21 patients (about 2%), of whom 10 had received immunosuppressive treatment without and 11 with cyclosporine. Taking into account the longer periods of observation of patients without compared with those with cyclosporine treatment (88 +/- 60 vs. 41 +/- 31 mo., respectively, P = 0.001), the two treatment groups did not differ significantly in prevalence of de novo MGN (4.0% vs. 1.5%). De novo MGN was diagnosed by biopsy 62.7 +/- 44.4 mo. after transplantation; its incidence increased significantly with time (from 0% to 5.3% over 8 years; 95% confidential interval: 1.7-8%). Proteinuria (mean, 3.2 +/- 2.9 g/L) was first observed 47.5 +/- 51.3 mo. after transplantation. Thirteen of the 21 patients (62%) were nephrotic (proteinuria, over 1.5 g/L). Steroid pulses were given to 12 patients with de novo MGN and high proteinuria, which did not decline after treatment. Signs of chronic viral infection (hepatitis B antigen, hepatitis C antibody, or human immunodeficiency virus antibody) were found in 8 of the 21 patients (38%). Signs of vascular or interstitial rejection were seen in 17 and 12 of the 21 patients with de novo MGN, respectively, and cyclosporine arteriolopathy was diagnosed in four. Graft loss occurred in 14 of the 21 patients and was due to rejection in 13 and to de novo MGN in only one, who developed additional transplant vein thrombosis. Patients with de novo MGN did not differ significantly from the other 851 patients in graft survival (71.4 +/- 9.9% vs. 60.8 +/- 2.2% after 5 yr). De novo MGN is a late, often asymptomatic, complication of initially well tolerated grafts and is neither prevented by cyclosporine treatment nor reversed by further steroid medication. It is often associated with vascular changes caused by rejection or cyclosporine toxicity.