Females have stronger immune responses than males. Female predominance in autoimmune diseases in both experimental animals and in man is also established. Sex hormones and some other steroid hormones have been shown to exert immunoregulatory effects in both in vivo and in vitro situations. Important mechanisms of hormone action on lymphocytes have gradually been revealed. In this paper the most intriguing effects of steroid hormones on immune system and present data suggesting hormonal influence on the pathogenesis of autoimmune diseases are summarized.