The three-dimensional structure of the ligand-binding region of human E-selectin has been determined at 2.0 angstrom resolution. The structure reveals limited contact between the two domains and a coordination of Ca2+ not predicted from other C-type lectins. Structure/function analysis indicates a defined region and specific amino-acid side chains that may be involved in ligand binding. These features of the E-selectin/ligand interaction have important implications for understanding the recruitment of leukocytes to sites of inflammation.