To investigate the role of mast cells during the process of tumor angiogenesis, we compared the rates of tumor vascularization, growth and metastasis in control WBB6F1 (-)+/+ mice and in their mast-cell-deficient WBB6F1-W/Wv littermates injected with MB49 murine bladder carcinoma cells. Our results demonstrate that in mast-cell-deficient mice injected with tumor cells, there is a decreased number of capillaries at the tumor periphery, reduced tumor size relative to control mice, and an absence of metastases. In contrast, when tumor cells were inoculated intravenously, both strains of mice showed high numbers of lung metastases. These results suggested that the reduction of blood vessels at the tumor periphery may lead to a reduction in the number of metastatic cells shed into the circulation in mast-cell-deficient mice.