Vascular endothelial (VE) cadherin is an endothelial specific cadherin that plays a major role in remodeling and maturation of vascular vessels. Recently, we presented evidence that the extracellular part of VE cadherin, which consists of five homologous modules, associates as a Ca(2+)-dependent hexamer in solution (Legrand, P., Bibert, S., Jaquinod, M., Ebel, C., Hewat, E., Vincent, F., Vanbelle, C., Concord, E., Vernet, T., and Gulino, D. (2001) J. Biol. Chem. 276, 3581-3588). In an effort to identify which extracellular modules are involved in the elaboration and stability of this hexameric structure, we expressed various VE cadherin-derived fragments overlapping individual or multiple successive modules as soluble proteins, purified each to homogeneity, and tested their propensity to self-associate. Altogether, the results demonstrate that, as their length increases, VE cadherin recombinant fragments generate increasingly complex self-associating structures; although single module fragments do not oligomerize, some two or three module-containing fragments self-assemble as dimers, and four module-containing fragments associate as hexamers. Our results also suggest that, before elaborating a hexameric structure, molecules of VE cadherin self-assemble as intermediate dimers. A synergy between the extracellular modules of VE cadherin is thus required to build homotypic interactions. Placed in a cellular context, this particular self-association mode may reflect the distinctive biological requirements imposed on VE cadherin at adherens junctions in the vascular endothelium.