Inflammatory cell infiltration around the sites of carcinoma invasion is believed to play important roles in tumor biological behavior. The status of inflammatory cell infiltration at the sites of frank invasion in 92 cases of gastric carcinomas was examined, with special emphasis on tumor-associated tissue eosinophilia (TATE). TATE was found in 7 out of 92 (7.6%) gastric carcinomas (6 of intestinal-type and 1 of diffuse-type). Electron microscopy, selectively performed in the 7 cases of gastric carcinomas with TATE, showed that eosinophils participated in the stromal reaction by interacting with tumor cells, mast cells, and each other. Most of the tumor-infiltrating mast cells exhibited anaphylactic or piecemeal degranulation, indicating that the mast cells had been activated in situ. Some mast cells were noted in close contact to viable tumor cells, suggesting the existence of direct cell-to-cell interactions. There was also extracellular deposition of free eosinophil granules and Charcot-Leyden crystals. These morphologic findings are similar to that described in late/chronic-phase allergic reaction in both human and experimental animals, where angiogenesis and fibrosis/tissue repair are also present. In conclusion, TATE may indicate a chronic allergic-like Th2 host-tumor reaction, and understanding these pathways should create tools to enhance defence and contrast neoplastic disease.