Flow visualization studies were conducted in a water-Plexiglas tube model which simulated flow conditions commonly encountered in major vessels. Complex secondary flow patterns in the main line downstream of a branching vessel were observed in found to be relatively independent of the diameter ratio and angle of departure of the branching tube. The ratio of branching to total flow was the prinicipal determinant of the aforementioned secondary flows. Venous and arterial branching markedly differed. Arterial-type flows produced a pair of vortex sinks downstream of the branching port. These vortex-sink flow fields, which previously have not been reported, captured fluid particles already downstream of the branching orifice and returned these particles upstream to exit via the branching tube. In a region very close to the tube wall, streamlines were also observed moving upstream to be captured by the branching vessel. The interaction of this local upstream flow with the general downstream flow produced a pair of stagnation points located downstream and lateral to the branching orifice. These stagnation regions are discussed in relation to atheroma and thrombus formation.