Ischemic renal disease is defined as a clinically significant reduction in glomerular filtration rate in patients with hemodynamically significant renal artery stenosis. The most common etiology for this is atherosclerotic renal artery disease. The three major clinical settings in which one must suspect ischemic renal disease include acute renal failure precipitated by the treatment of hypertension particularly with angiotensin-converting enzyme inhibitors; progressive azotemia in a patient with known renal vascular hypertension treated medically; and unexplained progressive azotemia in an elderly patient with refractory hypertension and other evidence of atherosclerotic disease. Prevalence of ischemic renal disease secondary to atherosclerosis can be estimated from the incidence of atherosclerotic renal artery lesions leading to renal vascular hypertension and the natural history of these lesions. Autopsy series, arteriography studies, and review of populations of patients in end-stage renal disease programs all suggest that ischemic renal disease has a high and increasing prevalence in our aging population.