The geometry of a typical adult human carotid bifurcation, complete with the sinus, was established from a study of a large number of angiograms. A rigid model was constructed from glass and investigations were performed under steady flow conditions using flow visualization techniques over a range of upstream Reynolds numbers and flow division ratios through the branches representative of physiologic conditions expected in the human vasculature. The study reveals a complex flow field in which secondary flows play an important role. The separation regions occuring at the outer corners of the branching are also zones of low wall shear stress but are not regions of recirculation. The apex and side walls appear to be subjected to much higher shear stress. Comparison with pathologic data on localization of atherosclerotic lesions indicates that zones susceptible to disease experience low or oscillatory shear stress while regions subject to higher shear are free of deposits.