Interleukins are mediators of inflammation, immunity and cancer. IL-15 is a cytokine produced by several leukocytes including phagocytes in response to infections and other signals that trigger innate immunity. IL-15 has many homologies to interleukin-2 (IL-2) and like IL-2, stimulates NK cells. This cytokine acts also on memory CD8+ T-cell. IL-15, therefore acts, probably through selective inhibition of tumor promoting molecules, as a new compound for the adjuvant treatment of solid tumors. In this review we propose a newly revised mechanism of interleukin 15 in inflammation and cancer.