Endometriosis is characterized by the presence of elevated proinflammatory cytokines such as tumor necrosis factor (TNF) alpha in the peritoneal cavity. Blocking interaction of TNFalpha with its receptor by the addition of excess TNFalpha-binding protein (TBP)-1 (a soluble form of TNF receptor-1) was effective in animal models of endometriosis. Recently, a novel, high-affinity inhibitor of TNFalpha, TNF-soluble high-affinity receptor complex (TNF-SHARC), was created by fusing TBP to both the alpha and beta subunits of inactive human chorionic gonadotropin. This dimeric protein was effective in inhibiting collagen-induced arthritis in mice. In the present study, the efficacy of TNF-SHARC in cellular and in vivo models of endometriosis was examined. TBP and TNF-SHARC dose-dependently inhibited TNFalpha-induced secretion of interleukin (IL)-6, IL-8, granulocyte macrophage-colony-stimulating factor, and monocyte chemoattractant protein-1 in immortalized human endometriotic cells. An in vivo mouse model of experimentally induced endometriosis using cycling C57BL/6 mice was established. Antide treatment (0.5 mg/kg), used as positive control, initiated 7 days after the establishment of the disease, reduced the weight of the lesions compared with control. TNF-SHARC at 3 mg/kg was not effective in inhibiting the disease, whereas at 9 mg/kg there was reduction in the lesion weight. In addition, antide and TNF-SHARC treatment in vivo increased in vitro natural killer cell activity compared with untreated animals. Thus, we provide evidence for supporting the development of TNF-SHARC as a therapeutic candidate for treating endometriosis in human.