The chemokine receptor CCR7 and its ligands regulate migration and colocalization of T cells and mature dendritic cells to and within secondary lymphoid organs. The requirement of CCR7 in efficient priming of allospecific cytotoxic CD8(+) T cells is poorly characterized. Here, we demonstrate a role for CCR7 in the initiation of an alloimmune response and in the development of transplant rejection. Remarkably, in a model of acute allogeneic tumor rejection, CCR7(-/-) mice completely failed to reject subcutaneously injected MHC class I mismatched tumor cells and cytotoxic activity of allospecific T cells was severely compromised. When solid tumors derived from wild-type mice were transplanted, recipient CCR7(-/-) mice were capable of rejecting the allografts. In contrast, tumor allografts transplanted from CCR7(-/-) donors onto CCR7(-/-) recipients showed allograft survival up to 28 days, suggesting a critical function of CCR7 on donor-type passenger leukocytes in the initiation of cytotoxic CD8(+) T cell responses. In a heterotopic heart transplantation model CCR7 deficiency resulted in significantly prolonged but not indefinite allograft survival. Additional prolongation of graft survival was observed when hearts from CCR7(-/-) mice were used as donor organs. Our results define a key role for CCR7 in allogeneic T cell priming within the context of draining lymph nodes.