Introduction: Leptin modulates immune activation in relation to nutritional state and there is gender difference in body composition and the immune response. Leptin induces the release of pro-inflammatory cytokines, including TNF-alpha, from human peripheral blood mononuclear cells (PBMCs). We hypothesised that estrogen may modulate the immune actions of leptin. Aim: To test the effect of estrogen on leptin signalling and leptin-induced TNF-alpha production. Materials and Methods: The impact of estrogen on leptin signalling was measured using a STAT3 responsive reporter plasmid (SIE-TK-luc) bioassay in the estrogen receptor positive MCF-7 cell line. To test the impact of estrogen on leptin-induced TNF-alpha production by PBMCs, FACS and ELISA analysis were performed. For FACS analysis, TNF-alpha immunostaining was measured 6 hours after treatment of whole blood with leptin and estrogen. In the ELISA analysis TNF-alpha was measured in cell conditioned medium of isolated PBMCs after 2 days incubation with estrogen and leptin. Results: Leptin induced a dose dependent increase in SIE activation, whereas estrogen alone had no effect. Estrogen addition in leptin-treated cells inhibited leptin-induced SIE activation (Fold induction 8.1 plus/minus 0.2 leptin vs 3.85 plus/minus 0.2 leptin plus estrogen). In human monocytes, FACS studies showed that estrogen alone had no effect on TNF-alpha production. In contrast, leptin induced TNF-alpha production. Co-treatment of leptin and estrogen inhibited the effect of leptin on TNF-alpha production (Fold induction 7.4 plus/minus 1.0 leptin vs 4.8 plus/minus 0.6 leptin plus estrogen). Similarly, the ELISA showed that leptin induces TNF-alpha release from PBMCs and co-treatment with estrogen inhibited leptin's action (Fold induction 2.6 plus/minus 0.2 leptin vs 1.9 plus/minus 0.2 leptin plus estrogen). Conclusion: Estrogen can inhibit the signalling and immune actions of leptin. This could be one factor explaining the sexual dimorphism observed in the immune response.