Various non-invasive techniques were used to study clinical and ultrasound aspects of extracranial arterial disease. cw Doppler was used prospectively to study the natural history of asymptomatic extracranial arterial disease (greater than 50% carotid stenosis, subclavian steal phenomena) in 401 patients. Whereas the cumulative death rate was high (44%) at 8 years, the stroke rate was low (6%). Progression of the vascular processes during follow-up correlated with later cerebrovascular events. Transcranial PW Doppler was used to study the intracranial collaterlisation in 151 asymptomatic patients with significant extracranial lesions. The majority of patients showed excellent collateralisation. The natural history of non-stenotic plaques (less than 30% carotid stenosis) was studied by means of high-resolution (10 MHz) duplex system analysis. Whilst most plaques remained constant during the subsequent follow-up period progression was seen in 24% and regression in 10%.