Adhesion molecules play an important role in vascular biology because they mediate vascular stability, permeability, and leukocyte trafficking to and from tissues. Performing microarray analyses, we have recently identified activated leukocyte cell adhesion molecule (ALCAM) as an inflammation-induced gene in lymphatic endothelial cells (LECs). ALCAM belongs to the immunoglobulin superfamily and engages in homophilic as well as heterophilic interactions. In this study, we found ALCAM to be expressed at the protein level in human and murine lymphatic and blood vascular endothelial cells in vitro and in the vasculature of human and murine tissues in vivo. Functional in vitro experiments revealed that ALCAM mediates adhesive interactions, migration, and tube formation in LECs, suggesting a role for ALCAM in lymphatic vessel (LV) stability and in lymphangiogenesis. Furthermore, ALCAM supported dendritic cell (DC) adhesion to lymphatic endothelium. In agreement with these findings, experiments performed in ALCAM mice detected reduced LEC numbers in various tissues and defects in the formation of an organized LV network. Moreover, DC migration from lung to draining lymph nodes was compromised in ALCAM mice. Collectively, our data reveal a novel role for ALCAM in stabilizing LEC-LEC interactions and in the organization and function of the LV network.