Diagnostic significance of flow separation within the carotid bifurcation demonstrated by digital subtraction angiography. The presence of an area of reversed blood flow due to flow separation in the interval carotid artery is a normal finding in Doppler ultrasound studies in vivo as well as in model carotid bifurcations. This flow separation phenomenon is caused by the geometry of the carotid bifurcation and the fluid dynamics of blood. We demonstrated the flow separation phenomenon on lateral-projection intra-arterial digital subtraction angiograms in 99 of 100 carotid bifurcations. The mean duration of flow separation was 5.8 seconds, with values up to 14 seconds in normal carotid bifurcations. The presence of this flow separation phenomenon is almost independent of atherosclerotic lesions and is not correlated with cerebral ischemia. Therefore, the angiographic finding of a flow separation has no diagnostic value. However, our results refer to a factor that may be important in the genesis of atherosclerosis at the carotid bifurcation, namely the duration of the stay of blood and its components within the area of flow separation. 