Mast cells have been observed in numerous types of tumors; however, their role in carcinogenesis remains poorly understood. The majority of epidemiological evidence suggests a negative association between the presence of mast cells and tumor progression in breast, lung and colonic neoplasms. Intestinal adenomas in the multiple intestinal neoplasia (Min, APC ^Min/+ ) mouse displayed increased numbers of mast cells and increased abundance of mast cell-associated proteinases as determined by transcriptional profiling with the Hu/Mu ProtIn microarray. To examine the role of mast cells in intestinal tumorigenesis, a mutant mouse line deficient in mast cells, Sash mice (c-kit ^W-sh/W-sh ), was crossed with the Min mouse, a genetic model of intestinal neoplasia. The resulting mast cell-deficient Min–Sash mice developed 50% more adenomas than littermate controls and the tumors were 33% larger in Min–Sash mice. Mast cell deficiency did not affect tumor cell proliferation; however, apoptosis was significantly inhibited in mast cell-deficient mice. Mast cells have been shown to act as critical upstream regulators of numerous inflammatory cells. Neutrophil, macrophage and T cell populations were similar between Min and Min–Sash mice; however, eosinophils were significantly less abundant in tumors obtained from Min–Sash animals. These results indicate a protective, antitumor role of mast cells in a genetic model of early-stage intestinal tumorigenesis.