Th17 are a newly discovered population of T helper lymphocytes characterized by the production of IL-17 (IL-17A). Interleukin 17 activates production of cytokines which are responsible for migration of neutrophils and development of inflammatory reaction. Among the other cytokines secreted by Th17 cells are TNF-a, IL-1b, IL-6 and IL-22. Development of Th17 lymphocytes gave rise numerous researches on mechanisms leading to differentiation of CD4+ cells towards new subpopulation and enabled to define factors conditioning the line. Murine Th17 lymphocytes differentiate in a specific cytokine environment which include TGF-b, IL-6 and IL-21, whereas human Th17 cell development requires TGF-b, IL-1b, IL-2 in combination with IL-6, IL-21, or IL-23. Th17 lymphocytes are currently the subject of intensive examinations concerning their role in the pathogenesis of many autoimmune, inflammatory and allergic diseases. More and more attention is directed into their role in anti-tumor immunity. Although the presence of Th17 lymphocytes in the ovarian cancer microenvironment has been proven, their influence on the proliferation of cancer cells is still not defined. Controversies around the role of Th17 in the pathogenesis of different kinds of tumor indicate that IL-17 functions either anti-tumor or, on the contrary, conduce the development of the tumor. Proinflammatory and angiogenesis are important mechanisms conditioning the proliferation of the tumor, in which the lymphocytes play the crucial role. Simultaneously, a lot of data indicate that increased expression of IL-17 can be in favour of anti-tumor function of Th17 lymphocytes. Profound cognition of the biology of Th17 lymphocytes, their differentiation and supression mechanisms require further reasearches, results of which enable to elaborate on new methods of anti-tumor therapy.