A growing body of evidence indicates a bi-directional relationship between the neuroendocrine system and immune functions. It is well known that lymphoid organs such the thymus, the spleen and peripheral blood produce growth hormone (GH) and GH receptor is expressed on different subpopulations of lymphocytes. Many in vitro and in animal studies demonstrate an important role of GH in immunoregulation. GH stimulates T and B cells proliferation and immunoglobulin synthesis, enhances the maturation of myeloid progenitor cells and is also able to modulate cytokine response. However, in humans GH deficiency (GHD) is not usually associated with immunodeficiency and only minor abnormalities of immune function have been reported, as compared to those observed in GHD animals. It is possible that in humans the GH produced locally in the immune system compensates for the lack of endocrine GH. In this review the main actions of GH on the immune system in vitro, in animal models and in humans are summarized.