The purpose of this study was to determine the prevalence, progression and prognosis of asymptomatic carotid artery stenosis in a population of 1198 patients with peripheral arterial disease (n = 986) or aortic aneurysm (n = 212), mean age 67.7 (S.D. = 10.0) years. Patients were recruited from 1985 to 1989 with annual assessment of carotid artery stenosis of over 50% using Doppler peak frequency analysis. Patients were followed up annually until 1990 or their first event, transient ischaemic attack (TIA), amaurosis fugax (AFx), stroke without antecedent TIA, or death (mean follow up 20 months). Life tables were used to determine risk of events in different patient groups. Only 164 (13.7%) patients had a stenosis of over 50% in either of the common or internal carotid arteries, disease was bilateral in 33 (2.8%) patients. A total of 33 patients (2.8%) had over 80% stenosis in common or internal carotid arteries. During follow up 37 (3.1%) patients developed a stenosis greater than 50%. Only 27 (2.3%) patients developed a stroke, 11 of which were fatal. A further 33 (2.8%) suffered a TIA or AFx and a total of 155 patients died during follow up. The total neurological event rate (TIA, AFx and stroke) was significantly associated with the presence of over 50% stenosis, [relative risk (RR) = 2.98, 95% confidence interval (95% C.I.) 1.68-5.29, p less than 0.001] and carotid bruit (RR = 1.16, 95% C.I. 1.23-3.81, p = 0.010). Although risk of stroke was higher in patients with a 50% stenosis, this failed to achieve statistical significance (RR = 1.78, 95% C.I. 0.66-4.80, p = 0.275).(ABSTRACT TRUNCATED AT 250 WORDS)