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) α in nine UC tissues and nine normal tissues were examined by immunohistochemistry. TNF-α 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-α proteins were more significantly detectable in UC tissues than in normal tissues. Furthermore, 65.2% of PAR2⁺ cells and 66.4% of TNF-α⁺ 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-α⁺ cells, respectively. A χ² analysis showed correlation (p < 0.007) between PAR2 and TNF-α in tryptase-positive mast cells. Moreover, PAR2 agonists significantly induced the TNF-α secretion from HMC-1. These results indicate that the activation of the mast cells through PAR2 may be involved in the pathogenesis of UC.