The significance of the finding of focal glomerulosclerosis (FGS) in idiopathic membranous glomerulonephritis (MGN) is uncertain. Twenty-seven patients with mixed FGS and MGN (MGN-FGS) were compared to 25 patients with MGN alone (generally matched for age, sex and stage of glomerular lesion) with respect to pathology, presenting clinical and laboratory features, and course of disease. Biopsies from the MGN-FGS patients showed significantly more extensive tubulointerstitial disease (P less than 0.001) than did those with MGN alone. At the time of biopsy, the MGN-FGS group had a significantly higher proportion of patients with hypertension (P = 0.006) and microhematuria (P = 0.006), a marginally higher percentage of patients with the nephrotic syndrome (P = 0.051), and a greater mean 24-hour urinary protein excretion (P = 0.004). A similar proportion of patients in each group were treated with either prednisone alone or prednisone with an immunosuppressive. Forty-eight percent of MGN-FGS patients and 13% of the MGN patients developed established renal failure in the follow-up period (P = 0.008). The renal survival rate for the MGN-FGS group was significantly lower at 24 months (0.61 vs. 0.93, P less than 0.05), 60 months (0.48 vs. 0.88, P less than 0.025), and over the entire follow-up period (P less than 0.05). The results indicate that FGS in MGN is associated with a significantly poorer prognosis than MGN without this lesion.