Systemic sclerosis (scleroderma) is a connective tissue disease characterized by excessive extracellular matrix deposition in the skin and visceral organs. Serum IL-6 levels are reported to be elevated in human scleroderma and chronic graft-versus-host disease (cGVHD) patients. IL-6 blockade using anti-IL-6 receptor mAb (anti-IL-6R mAb) results in amelioration of the pathologic symptoms of some autoimmune diseases such as rheumatoid arthritis and juvenile idiopathic arthritis. In this study, we examined the effects of anti-IL-6R mAb on either prevention or treatment of murine sclerodermatous cGVHD (Scl-cGVHD). We found that serum IL-6 levels in Scl-cGVHD mice gradually increased after bone marrow transplantation. Administration of anti-IL-6R mAb attenuated the development of severe Scl-cGVHD and fibrosis and resulted in an increase in CD4(+)CD25(+)FoxP3(+) regulatory T cells. However, treatment of established Scl-cGVHD with anti-IL-6R mAb showed no effects on disease severity. The effects of anti-IL-6R mAb were mostly inhibited by anti-CD25 mAb. Together, our results indicate that IL-6 has an important role in the pathogenesis of Scl-cGVHD. IL-6 blockade may be an effective approach for preventing Scl-cGVHD and treating cGVHD and scleroderma in humans.