The work deals with contemporary opinions on the endocrinological function of thymus. The latest information concerning T cell maturation has also been provided. It has been shown that there is a direct relationship between the process of lymphocyte T proliferation and formation of the organism tolerance to self. The current opinions dealing with the phenomenon of the thymus involution have been given stressing its advantageous significance for human organism. The work shows wide connections of the gland in question with other organs. The participation of thymus in pathogenesis of some disease entities has also been presented.