In order to assess the prevalence of carotid artery disease in patients with arteriosclerosis obliterans (ASO) or aortic aneurysm, 142 patients underwent screening for the presence of carotid atherosclerosis (plaque) with B-mode ultrasonography. Seventy-four (87.1%) of the 85 patients with ASO and 37 (64.9%) of the 57 patients with aortic aneurysm had carotid artery lesions. Of these patients, about half had bilateral lesions. The differences in the prevalence of carotid lesions were not significantly related to age or sex. Prevalence did not differ in relation to the severity or location of the primary lesions. The majority of the carotid lesions were located in the internal carotid arteries. The prevalence of carotid lesions was higher among patients with cerebral infarction than in those without any lesions on computed tomography. These results showing high prevalence of carotid lesions in ASO and aortic aneurysm suggest the usefulness of routine carotid ultrasonographic screening and also that it is important in order to determine the correct treatment for these cases.