Wall shear stress has been implicated in the genesis of atherosclerosis because a strong correlation exists between the location of developing arterial lesions and regions where particular gradients in stress occur. Studying the behavior of endothelial cells in such regions may contribute to our understanding of the disease entiolgy. We report the detailed migratory of endothelial cells subjected to large shear stress gradients caused by a surface protuberance in an in vitro model system. The history of cell migration, cell division, and cell loss from the surface was continuously monitored in confluent human umbilical vein endothelial cell monolayers for 48 hours after the onset of flow. Individual cells were tracked using time- lapse video microscopy. In contrast to a uniform laminar flow field in which cells were observed to continually rearrange their relative position with no net migration, in a disturbed flow field there was a net migration directed away from the region of high shear gradient. This organized migration pattern under disturbed flow conditions was accompanied by more a twofold increase in cell motility. In addition, cell division increased in the vicinity of the flow separation (maximum shear stress gradient of 34 dyne/cm2 per mm) whereas cell loss wa increased upstream and downstream in the regions where the shear gradient diminishes. These data suggest a steady cell proliferation-loss cycle and indicate that local shear gradients may play a key role in the morphological remodeling of the vascular endothelium in vivo.