We previously showed that renal artery stenosis (RAS) was commonly found in patients with cardiovascular disease (CVD) such as myocardial infarction, stroke, or abdominal aneurysm. The aim of the present study was to evaluate the incidence and risk factors for RAS in patients with severe carotid artery stenosis (CAS) considered to need carotid endarterectomy. From February to August 2006, 41 consecutive patients with severe CAS were admitted to the Department of Neurosurgery of the National Cardiovascular Center. Each patient was examined for renal function and urinary albumin excretion, and renal artery duplex scanning was also performed. The patients were classified into two groups according to the findings of renal Doppler sonography, 11 patients with RAS and 30 patients without RAS. We evaluated the differences in clinical findings and renal function between the groups and clarified the risk factors for RAS. In RAS patients, smoking and incidence of other CVDs were evident, and renal function was impaired significantly compared with the patients without RAS. Multivariate logistic regression showed that the presence of other CVDs, renal function, and smoking were significant clinical predictors for RAS. In patients with severe CAS, RAS was frequently detected with the same frequency as ischemic heart disease. The RAS risk factors were the presence of other CVDs, renal dysfunction, and smoking. Since RAS is an underlying cause of hypertension and renal failure, it is important to consider the presence of RAS in patients with severe CAS.