Interleukin 33 (IL-33) is a recently discovered cytokine member of the IL-1 superfamily that is widely expressed in fixed tissue cells, including endothelial and epithelial cells. IL-33 induces helper T cells, mast cells, eosinophils, and basophils to produce type-2 cytokines through binding to the ST2/IL-1 receptor accessory protein complex. Recent studies have shown IL-33 to be upregulated in intestinal parasite infection and in epithelial cells and myofibroblasts in ulcerative colitis (UC). The findings point to a role for IL-33 in directing the T(H)2-type immune responses in these types of mucosal inflammation. As the IL-33/ST2 receptor axis can be manipulated by various blocking antibodies, this could be a potential therapeutic target in the future treatment of UC.