We analyzed clinicopathologic characteristics of adult-onset membranoproliferative glomerulonephritis (MPGN) by comparing two tentatively-classified subgroups. Group A consisted of 9 patients in whom more than 50% of glomeruli showed mixed segmental and global duplication of glomerular basement membrane (GBM), and Group B 9 patients with global duplication of GBM. Group A showed a tendency for more favorable clinical course and outcome than Group B. Nephrotic syndrome was present in 33% of Group A and 100% in Group B, hypertension in 22% and 44%, hypocomplementemia in 44% and 67% at the time of renal biopsy. Deterioration of renal function, at comparable durations of follow-up of 62 +/- 12 (M +/- SE) and 53 +/- 13 months, was observed in 22% of Group A and 56% in Group B, respectively. Histologically, mesangial proliferation and tubulointerstitial change were more pronounced and frequency of sclerosing glomeruli was greater in Group B. There was also a negative correlation between the extent of global double contour and renal function as assessed by creatinine clearance at the time of renal biopsy (r = -0.55, P less than 0.05). These results indicate that the high incidence of global double contour, in addition to the presence of marked tubulointerstitial change and sclerosing glomeruli, may relate to progressive deterioration of renal function in MPGN. That outcomes for Group A and Group B may be different when clinical parameters, including the durations from onset of symptoms to the time of biopsy and length of follow-up periods, are comparable also indicates that these two groups should be considered separate entities, at least on the basis of clinical results.