Cellular mechanisms underlying sexual dimorphism in the immune response remain largely unknown. Concerning the interactions among the nervous, endocrine and immune systems, we reported that during gestation, a period during which multiple sclerosis (MS) clearly ameliorates, there is a physiological expansion of regulatory T-lymphocytes (T_Reg). Given that alterations in T_Reg proportions and suppressive function are involved in MS pathophysiology, we investigated the in vitro effect of sex hormones on T_Reg. Here, we show that both E2 and progesterone (P2) enhance T_Reg function in vitro, although only E2 further induces a T_Reg phenotype in activated responder T-cells (CD4⁺CD25⁻) (P < 0.01). E2 receptor beta (ERβ) percentages and mean fluorescence intensity (MFI) on T_Reg were lower in MS patients than in controls (P < 0.05), in parallel with lower E2 plasma levels (P < 0.05). Importantly, percentages and MFI of ERβ were higher in T_Reg than in T-responder cells (P < 0.0001) both in MS patients and controls. We show a unique differential pattern of higher ER and PR levels in T_Reg, which may be relevant for the in vivo responsiveness of these cells to sex hormones and hence to MS physiopathology.