Experimental and clinical evidence suggest that immune reactivity is modulated by gender. Immune reactivity is greater in females than in males and lymphocytes and monocytes from female subjects shows higher antigen presenting activity and mitogenic responses. Steroid hormones can be converted along defined pathways to downstream hormones in the periphery. The conversion of dehydroepiandrosterone (DHEA) in target macrophages leads to an increase of downstream effector hormones (including estrogens), which may be an important factor for local immunomodulation at least in RA synovitis. The presence in the RA synovial fuids (SF) of an altered sex hormone balance resulting in lower immunosuppressive androgens and higher immunoenhancing estrogens, might determine a favorable condition for the development of the immuno-mediated RA synovitis and synovial hyperplasia. The increased estrogen concentration observed in RA SF of both sexes are characterized by the hydroxylated forms, in particular 16α-hydroxyestrone, that is a mitogenic and proliferative endogenous hormone. In contrast to 16α-hydroxylated estrogens, the 2-hydroxylated forms inhibit growth promoting effects of E2 and were found low in RA SF. Therefore, dose-related conversion to pro- or anti-inflammatory dowstream metabolites of estrogens might support the dual role of estrogens (pro or anti-inflammatory) for example during estrogen replacement therapy, depending on local concentration (i.e. SF in RA) of 16α-hydroxyestrone or 2-hydroxyestrogens.