A fine structure of blood flow through a curved vessel with an aneurysm was studied in in vitro experiments in relation to rheological factors of arterial diseases such as arteriosclerosis or thrombosis. On the basis of the in vivo data related to cerebral circulation, red blood cell suspension was flowed through curved vessel models with an asymmetrical aneurysm. Flow visualization was made with a microscope 16 mm cinecamera-TV monitor system, and the velocity profile was measured using the laser Doppler velocimeter. Vortices induced in aneurysm influenced flow structure and velocity at the presence of the secondary flow due to the vessel curvature. This suggests strongly that blood flow in curved arteries with an aneurysm must be understood under the influence of the secondary flow.