Recent reports suggested that the presence of terminal complement complex (C5b-9) in urine from patients with idiopathic membranous nephropathy (IMN) may indicate on-going immunological damage. This report documents the relationship between C5b-9 excretion and clinical outcome in 35 adult patients with biopsy-proven IMN and progressively declining renal function. There were two groups of patients. Group I received one of three treatment regimens: prednisolone alone, prednisolone and chlorambucil, or prednisolone and cyclophosphamide (N = 22). Group II received no immunosuppressive therapy (N = 17). Three of the 18 patients receiving immunosuppressive drugs had more than one treatment regimen as they experienced a clinical relapse during the study period; hence 22 treatments were available for analysis. Urine samples were collected regularly and urinary C5b-9 (uC5b-9) was determined by ELISA. Both groups were similar with respect to age, sex distribution, and the duration of follow-up. An improvement in proteinuria and creatinine clearance was noted in the immunosuppressed group. Thirty-five patients were excreting C5b-9 initially (18 from group I and 17 from group II); 17 patients continued to excrete C5b-9 at the end of the observation period. These 17 patients had a significantly worse clinical outcome when compared to the 18 patients whose C5b-9 excretion became negative, either spontaneously or with treatment (P < 0.005). These results indicate that continuing C5b-9 excretion is correlated with a poor clinical outcome. They also suggest that uC5b-9 is a dynamic marker of ongoing immunological injury, and therefore may be useful in the initial assessment and monitoring of patients with IMN and in identifying patients who may derive benefit from immunosuppressive therapy.