Biopsies of tumors responding to interleukin 2 (IL-2) based immunotherapy have been reported to show a leukocytic infiltration. Clinical responses to IL-2-based immunotherapy, however, are limited, suggesting a failure of leukocyte localization at tumor sites. Leukocyte infiltration at inflammatory sites requires local activation of leukocytes and endothelial cells in a coordinated and defined temporal sequence. There is evidence supporting the theory that infiltration of leukocytes at tumor sites is suboptimal due to a failure of coordination of these localizing events. In this review, factors involved in leukocyte recruitment at sites of inflammation and the coordination of these factors in a successful model of inflammation, i.e., wound healing, are discussed. This example is contrasted with events at tumor sites where alterations in expression of cell adhesion molecules or in the production of activating agents may be present. Additionally, the systemic administration of an activating cytokine such as IL-2 may fail to duplicate events that normally occur within a local environment. These observations may facilitate the design of future immunotherapy trials.