Eight patients with idiopathic membranous nephropathy whose renal function was deteriorating were given a 6-month course of alternating monthly cycles of prednisolone and chlorambucil. Proteinuria was reduced in all eight, from a mean (SD) of 15.3 (5.9) g/24 h at the start of treatment to 2.1 (1.5) g/24 h at follow-up (p less than 0.05). Creatinine clearance increased in six, and the rate of decline was reduced in the other two (group mean 51.6 [17.8] ml/min at the start of treatment and 81.4 [36.8] ml/min at follow-up; p less than 0.05). Adverse effects of chlorambucil were severe, and the daily dose had to be reduced. Prednisolone and chlorambucil treatment can change the natural course of membranous nephropathy even when renal function has started to deteriorate, so treatment can be reserved for high-risk patients.