The association of mast cells with tumor angiogenesis was investigated in patients with esophageal squamous cell carcinoma. Surgical specimens from 48 patients with esophageal squamous cell carcinoma were studied. Mast cells in tumor sections were stained with Alcian blue and safranin O. The number of mast cells was counted under light microscopy and the average count recorded. To highlight the microvessels, endothelial cells were stained with anti-human factor VIII antibody. Microvessel density was also counted. We found a significant correlation between mast cell count and microvessel density in patients with esophageal squamous cell carcinoma. Double staining of the microvessels revealed highly angiogenic areas densely populated with mast cells. There appears to be a direct correlation between the number of mast cells and tumor angiogenesis in patients with esophageal squamous cell carcinoma.