Although tryptase released from mast cells might play a key role in the pathogenesis of ulcerative colitis (UC), the role of protease-activated receptor 2 (PAR2), tryptase receptor, remains unclear in the pathogenesis of this disease. The expressions of PAR2 and tumor necrosis factor (TNF) alpha in nine UC tissues and nine normal tissues were examined by immunohistochemistry. TNF-alpha levels secreted from human leukemic mast cell line (HMC-1) after the treatment of PAR2 agonists were also measured by enzyme-linked immunosorbent assay. The PAR2 and TNF-alpha proteins were more significantly detectable in UC tissues than in normal tissues. Furthermore, 65.2% of PAR2+ cells and 66.4% of TNF-alpha+ cells in UC tissues were tryptase-positive cells. In other words, 60.6% and 46.3% of tryptase-positive cells in UC tissues were PAR2+ cells and TNF-alpha+ cells, respectively. A chi2 analysis showed correlation (p < 0.007) between PAR2 and TNF-alpha in tryptase-positive mast cells. Moreover, PAR2 agonists significantly induced the TNF-alpha secretion from HMC-1. These results indicate that the activation of the mast cells through PAR2 may be involved in the pathogenesis of UC.