Genetic analyses of cancer in humans indicate that chemokines and their receptors are unlikely to play direct roles in pathogenesis. However, these molecules have pleiotropic effects that impact on cancer pathobiology in animal models, and there is evidence that they may do the same in humans. Given their protean properties, chemokines could have tumor-promoting, tumor-suppressing activities, or either depending on context. An example is found in CCL2, a chemokine that attracts and activates mononuclear cells. In some settings, it stimulates host anti-tumor activities. However, tumor cells themselves secrete CCL2 suggesting that it has growth promoting effects. These have been documented in animal models and clinical epidemiological studies. If CCL2's protumorigenic activities can be validated, then CCL2 and its receptor CCR2 may be therapeutic targets in cancer.