BACKGROUND: Increased numbers of mast cells are found in various solid tumors. To investigate the role of mast cells in the vicinity of gastric cancer cells, we used special staining and an immunohistochemical technique.METHODS: Specimens were surgically obtained from 102 patients with gastric cancer. Mast cells around the tumor edge of gastric cancer nests were counted by staining with 0.05% toluidine blue solution. Blood vessels in these areas were also counted, by immunohistochemical staining of endothelial cells for factor VIII.RESULTS: The average number of mast cells and blood vessels in gastric cancer specimens was significantly higher than that in normal gastric tissue. Specimens from patients with advanced disease with metastases to lymph nodes had more mast cells than specimens from patients with early-stage disease. Mast cells in specimens from patients with metastatic lymph nodes were significantly increased in comparison with numbers in specimens from those without nodal metastases. Mast cell numbers in the specimens of patients with lymphatic or blood vessel invasion were significantly higher than numbers in specimens from patients without such invasion. Mast cells were localized near the new vessels around gastric cancer cells. Mast cell numbers increased as the number of blood vessels increased (correlation coefficient, 0.783). Postoperative survival curves revealed that patients with increased numbers of mast cells had a poor prognosis.CONCLUSIONS: All these results suggest that mast cell accumulation at the tumor site may lead to increased rates of tumor vascularization and, consequently, increased rates of tumor growth and metastasis.