In vivo studies in experimental animals and in vitro studies using human lymphocytes have indicated that growth hormone (GH) is important for the development and function of the immune system. This is most clearly illustrated by the underdevelopment of the thymus in dwarf Snell mice, which is prevented by GH treatment. In contrast, in vivo studies in humans have generally shown only minor alterations of immune function in GH deficiency. GH administration to children with various growth disorders may lead to variable and subtle changes in some laboratory parameters of immune function, but no clinical symptoms associated with immune dysfunction have been reported.