Objective: To evaluate the T helper 1 (T H 1)/T helper 2 (T H 2) lymphocyte cytokine pro®les in women and men and to study the in vitro effects of sex hormones on lymphocyte secretion of cytokines. Methods: Analysis of serum concentration and lymphocyte synthesis of T H 1 (gamma interferon (INF-g) and interleukin 2 (IL-2)) and T H 2 (interleukin 4 (IL-4) and interleukin 10 (IL-10)) cytokines was performed in 20 women and 15 men. Analysis of modi®cations in cytokine secretion induced by supplementation of lymphocyte culture with increasing concentrations of sex hormones was carried out. Results: Higher levels of INF-g and IL-2 and lower levels of IL-4 and IL-10 were detected in the phytohemagglutinin-stimulated lymphocyte culture supernatants of men compared with women; the INF-g:IL-4 ratio was signi®cantly higher in men. In women, similar concentrations of all the cytokines were detected in culture supernatants obtained during the follicular and the luteal phases. The addition of sex hormones did not modify the concentration of cytokines in supernatants of phytohemagglutinin-stimulated T-cell cultures.Conclusions: Women present a predominant T H 2 cytokine pro®le, which could be involved in immune responses characterized principally by the secretion of antibodies. This could be a factor implicated in the higher concentration of immunoglobulins or the increased prevalence of autoimmune diseases detected in females.