We report a patient with diffuse peritoneal metastases of GIST who was administered STI571 (a tyrosine kinase inhibitor). She was a 45-year-old woman who underwent surgery for a mass that was suspected to be an ovarian tumor. The postoperative diagnosis was GIST developing from the small intestine. Two years later, multiple peritoneal metastases were noted and a complete macroscopic resection was done. Six months after the second operation, she needed a third operation and complete macroscopic resection was repeated. Only 4 months after the third operation, she underwent a fourth operation. We resected massive peritoneal metastases, but diffuse metastases remained. Therefore, we started to administer STI571 at 400 mg/day. She has been free from peritoneal masses for 9 months after the fourth operation. We recommend STI571 for uncontrolled diffuse peritoneal metastases of GIST.