During a 9-month period the carotid arteries of 2420 consecutive patients were investigated by conventional Doppler and duplex ultrasonography as well as by colour-coded duplex sonography. Of 186 internal carotid arteries showing a diameter reduction at the bifurcation level of 95% or more by conventional techniques, 131 underwent X-ray angiography which revealed a subtotal stenosis in 25 and an occlusion in 106 cases. Using a total of ten criteria for describing the colour-coded duplex findings, the "distal colour filling", a combination of three single criteria, could be found in all subtotal stenoses and none of the occlusions. "Distal colour filling" was diagnosed if a colour signal was present for a length of at least 1.5 cm directly behind an assumed stenosis comprising the whole area between the vessel walls. Our results suggest that non-invasive colour-coded duplex sonography is superior to conventional Doppler and duplex techniques and comparable to invasive X-ray angiography in differentiating extracranial subtotal carotid artery stenoses from occlusions.