In a series of 60 patients with idiopathic membranous nephropathy (IMN), 8 subjects, aged 16-65 years at presentation, suffered spontaneous relapse of proteinuria after remissions of 25 months to 30 years. Renal biopsy was performed at the time of relapse in 5 cases and revealed histopathology identical to that of the original lesion. Eight courses of immunosuppressive therapy given to 6 patients did not affect either the appearance or duration of remission or relapse. No patient had a familial tendency to renal disease. Immunogenetic markers, HLA A, B and DR, did not distinguish those who relapsed from other patients with IMN. At the end of the study, 3 patients were in a second remission, one had died of myocardial infarction during relapse, 3 remained nephrotic and one had mild renal insufficiency but no proteinuria. As compared with the rest of the series the overall prognosis was not influenced by the relapses.