In membranous nephropathy it would be of great value to be able to identify in an early phase patients at highest risk for disease progression, since potentially toxic treatment could then be restricted to these patients only. We measured renal hemodynamics, serum proteins and urinary protein excretion in 22 patients with membranous nephropathy, nephrotic syndrome and normal renal function (endogenous creatinine clearance > 85 ml/min). These patients were followed for a mean of 56 months. Renal function deteriorated in nine patients. When using univariate analysis, deterioration of renal) function appeared to be associated with a low serum albumin and transferrin, high urinary transferrin, beta-microglobulin, and IgG excretion, but not with renal hemodynamics. A step-up procedure, used for selecting variables associated with survival, showed that IgG excretion was independently associated with renal function deterioration. In patients with membranous nephropathy and normal renal function, the urinary excretion rate of IgG predicts future renal function outcome.