A 65-year-old man was referred to our hospital for evaluation of his huge abdominal tumor. He was diagnosed as having a gastrointestinal stromal tumor arising from the stomach. Seven months after surgery, multiple liver metastases and mesenteric dissemination occurred. He was medicated with STI571, which works by blocking proliferation of malignant cells with expression of c-kit. The tumors shrank and serum lactate dehydrogenase and alkaline phosphatase concentrations fell to below the normal limit three months later. STI571 was effective medicine for the metastatic gastrointestinal stromal tumor for six months in this case.