Vascular endothelial cells are an interface between blood vessel walls and blood flow, and play important roles in physiological functions. Since endothelial cell responses to fluid shear stress have been implicated in the localization of atherosclerosis, the effect of shear stress on endothelial cell morphology and functions has been exclusively studied. After applying fluid shear stress, cultured endothelial cells show marked elongation and orientation in the flow direction. In addition, thick stress fibers of actin filaments appear and align along the cell long axis. Thus, the endothelial cell morphology is closely related to the cytoskeletal structure. The purpose of this review is to summarize endothelial cell responses to fluid flow which have been studied, focusing on the changes in cell shape and cytoskeletal structure. Numerical studies to simulate local flow field at the cellular level and the resulting intracellular stresses are also reviewed. (C) 2005 Published by The Japan Society of Fluid Mechanics and Elsevier B.V. All rights reserved.