A 70 year old patient with malignant hypertension due to bilateral renal artery disease developed acute renal failure within 36 hrs of starting an angiotensin converting enzyme (ACE) inhibitor. Review of the consecutive series of 14 patients with bilateral renal artery disease who had also been given ACE inhibitors showed that functional renal sufficiency was common but not invariable under these circumstances. Although renal function returned towards pretreatment levels in most cases when ACE inhibitors were withdrawn, the speed with which acute renal failure may occasionally develop suggests that renal artery disease should be excluded before prescribing ACE inhibitors for hypertensive patients with undiagnosed renal impairment.