A 73-year-old man was referred to our hospital because of positive results on a fecal occult blood test. He had severe anemia, and abdominal computed tomography (CT) revealed a huge 18 × 11 cm tumor in the lower gastric corpus. Pathological analysis of a biopsy sample revealed a gastrointestinal stromal tumor (GIST). We improved the nutritional and physical statuses via total parenteral and enteral nutrition. At the same time, we initiated preoperative adjuvant chemotherapy with 300 mg/day of imatinib. After 4 weeks' of treatment, CT revealed a drastic reduction in the tumor size, regarded a partial response (PR). The patient underwent partial gastrectomy, distal pancreatectomy, and partial resection of the transverse colon. His postoperative course was uneventful. The patient received postoperative treatment with imatinib for 1 year, and remained recurrence free for 10 months after surgery.