Cytokines are key modulators of host immune and inflammatory responses. The expression of cytokine genes by tumor cells as a result of gene transfer has emerged as a novel strategy to augment in vivo host reactivity to various cancers. This review summarizes the knowledge obtained from experimental systems using this strategy and provides information on the current clinical trials employing this approach. In murine model systems, immunization with tumors expressing certain cytokines [e.g., tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), interleukin-7 (IL-7), and granulocyte-macrophage colony stimulating (GM-CSF)] has demonstrated their ability to promote the generation of tumor-specific cytotoxic T lymphocytes by various mechanisms; in some cases, significant regressions of established microscopic tumor deposits result. Non T cell mechanisms of tumor killing, such as granulocytic inflammatory responses, may also be elicited by the localized elaboration of certain cytokines [e.g., IL-4, granulocyte colony-stimulating factor (G-CSF)]. The potency of antitumor immune potentiation by cytokines, however, remains to be established by further animal studies and emerging clinical trials. The genetic modification of tumors for the expression of immunostimulatory gene products holds promise as a new approach for active immunotherapy of cancer and for the isolation of effector cell populations for use in adoptive immunotherapy protocols.