Previous research in cellular adhesion has focused primarily on studying isolated cells under conditions where cells do not interact with each other. However, in vivo cells form sheets where both cell-substratum and cell-cell interactions contribute to the overall adhesive behavior. Our understanding of how cell-cell and cell-substratum interactions affect the overall process of cell adhesion in these situations is limited. To address this problem, we developed a systematic approach to evaluate how cell-cell and cell-substratum interactions affect the critical shear stress for detachment for semi-confluent and confluent sheets of cells. Our studies were based on subjecting cultures of adherent cells to a defined hydrodynamic flow in a radial-flow chamber with a gap height of 140 microm. Using phase-contrast microscope imaging and analysis we measured shear-dependent patterns of detachment as a function of the extent of cell confluency. Our results show that the critical shear stress for detachment is maximum at intermediate extents of confluency of 10%-40%. These results have important implications for sodding vascular grafts and tissue engineering.