Abstract. Fifteen patients presented between January 1986 and January 1991 with deterioration in renal function coincident with the introduction of angiotensin converting enzyme inhibitors. There was evidence of extrarenal vascular disease in 12 patients and preexisting renal impairment in 13. Four patients remained dialysis-dependent and died within 4 weeks of presentation. Five patients required short-term dialysis. Serum creatinine remained above pre-treatment values in seven patients. Conventional explanations of the decline in renal function with ACE inhibition do not account for irreversible decrements in renal function. Possible mechanisms for this observation and clinical guidelines to identify patients at risk are suggested. We conclude that these agents should be used with great care in patients in whom atherosclerotic vascular disease is likely.