Mast cell sarcoma is a rare disease. We report an unusual case of this neoplasm arising in the ascending colon of a 32-year-old Japanese woman who presented with abdominal pain. An ulcerating mass in the colon was resected, along with enlarged mesenteric lymph nodes. Two years after surgery, the neoplasm recurred as left cervical lymphadenopathy and an intra-abdominal mass. Despite predonine and radiation therapy, the disease progressed, and the patient died. The tumor cells had abundant fine granular or clear cytoplasm, and oval, lobulated, or indented nuclei. Numerous mature eosinophils were intermingled with the tumor cells. Immunohistologic studies on paraffin sections demonstrated that the majority of the tumor cells were strongly positive for CD45RB, CD68, and mast cell tryptase. They were unreactive, however, with a broad spectrum of antibodies against myelomonocytic and lymphocytic antigens. The mast cell nature of this rare type of tumor can be best identifiable by immunostains for mast cell tryptase.