The chemokine system has evolved primarily to control the trafficking of leukocytes during immune or inflammatory responses. However, through their expression of chemokine ligands and receptors, cancers have commandeered various aspects of this host defence system in order to enhance their growth. Although engineered over-expression of some tumour-derived chemokines can stimulate host antitumour respones, this is unlikely to be the reason that tumour cells express them. Rather, a growing body of clinical and laboratory evidence indicates that cancer cells may secrete chemokines in order to attract host cells that supply the tumours with growth and angiogenic factors. In addition, chemokine receptor expression by tumour cells may permit them to use the host's pre-existing leukocyte trafficking system to invade target tissues during metastatic spread. Together, these observations suggest that therapies directed against chemokine ligands or receptors may be beneficial in cancer.