Interleukin-10 is an important cytokine that is involved in regulation of pro-inflammatory cytokines and T-cell responses. Interleukin-10 has been studied extensively in various preclinical and clinical models of inflammation. The most remarkable and consistently reproducible quality of IL-10 is its ability to downregulate macrophage functions. This includes inhibiting the production of pro-inflammatory cytokines such TNF-alpha, Interleukin-1, Interleukin-6 and antigen presentation by these professional antigen presenting cells. Additionally, Interleukin-10 also has effects on various other cell types of hematopoietic origin such as B-cells, neutrophils, and most importantly T-cells. Interleukin-10 has shown efficacy in several models of autoimmune disease. The present article deals with the effect of Interleukin-10 in animal models of inflammatory bowel disease and the results of phase I clinical trials in normal human volunteers and chronic active Crohn's disease patients.