At a two years' interval, a patient with severe hypertension was treated with captopril and MK 521, two different angiotensin converting enzyme inhibitors. Each time the blood pressure decreased to normal levels after addition of diuretic therapy but the patient developed acute renal failure. On both occasions the interruption of the inhibitors restored renal function very rapidly. In this woman, considered to suffer from essential hypertension on the basis of a normal intravenous pyelogram, arteriography showed bilateral renal-artery stenosis.