Interleukin 15 (IL-15) is a cytokine with strong impact on immunological system, that affects pathogenesis of multiple human disorders. Naturally, it participates in antiviral defence. Infected peripheral blood mononuclear cells secrete IL-15 which, via its impact on NK cells and T lymphocytes, exerts antiviral properties. IL-15 may find its application in HIV infection treatment. Stimulating proliferation of antigen- activated T cells and inhibiting their apoptosis, IL-15 may counteract AIDS-associated lymphocytopaenia. It may directly and indirectly influence anti-HIV reactions: enhancing CD8+ T cell and NK cell responses as well as increasing secretion of chemokines, which act competitively with HIV molecules. However, there are several concerns regarding IL-15 as factor stimulating HIV replication. IL-15 plays an important role in many autoimmune processes. In rheumatoid arthritis, IL-15 is extensively secreted by synoviocytes. It supports chemotaxis of inflammatory cells to affected joints and directly stimulates secretion of proinflam- matory cytokines. Novel therapeutic strategies for rheumatoid arthritis may involve inhibition of IL-15 function. IL-15 seems to play an important role in the pathogenesis of other autoimmune disorders such as colitis ulcerosa, Crohn`s disease, multiple sclerosis and systemic lupus erythematosus. Similarly, IL-15 may contribute to the process of solid organ transplant rejection and may constitute for a target of future immunosuppressive therapies. In case of stem cell transplantation, IL-15 may exacerbate graft-versus- host reaction, but also enhance graft-versus-tumour reaction and shorten time to reneval of immunological parameters. During allergic reactions, IL-15 seems to inhibit acute response, but also potentiates chronic reactions. The article describes results of former investigations on IL-15 functions in pathological proces- ses and resulting possibilities of therapeutic approaches.