Dye flow patterns were studied in 12 glass model bifurcations with angles of 45, 90, 135, and 180°, and area ratios of 0.78,1.03 and 1.27. At the apex, the dye formed a saddle zone, and streamlines from the core which entered this region were swept over the upper and lower surfaces to enter the lateral angles. Qualitatively, the shape and size of the apex played a key role in this effect. Boundary layer separation occurred in the lateral angles, and increased as flow into the branch was reduced. If the branch was occluded, acomplex vortex developed in the first few diameters of the branch, and no flow occurred beyond this, even though the occlusion was about 20 diameters downstream. The results were comparable with steady and pulsatile flow. The implications of these results for the localization of atherosclerosis are discussed.