Gastrointestinalstromaltumors (GIST)are occasionally found as large abdominal tumors at presentation. However, the best treatment approach for such large and marginally resectable GISTs remains unclear. The survival of patients with clinically malignant GISTs or GISTs with incomplete resection was as short as 1-2 years. Imatinib, a KIT kinase inhibitor, shows promise as a preoperative treatment for marginally resectable GIST, because it exhibits potent antitumor activity for unresectable and metastatic GISTs. Data obtained from imatinib therapy for advanced GISTs indicate that preoperative treatment with 400 mg of imatinib daily for 6-12 months is recommended, although no standard regimen has been established so far. Positron emission tomography is useful for the early assessment of the efficacy of preoperative imatinib treatment, a critical step for the management of patients with marginally resectable GIST. Two case studies have shown the safety and strong antitumor activity of preoperative imatinib treatment and concluded that treatment contributed to reducing surgical morbidity. However, a multicenter phase II trial conducted in the United States has shown that complete resection was not sufficiently achieved in patients who underwent preoperative imatinib treatment: complete resection rates were 77% for primary cases and 58% for metastatic cases, whereas the treatment was not associated with severe postoperative complications. The clinical guidelines for GIST in Japan regard preoperative imatinib treatment for marginally resectable GIST as an experimental treatment. More clinical evidence is required before making preoperative imatinib treatment the standard treatment for marginally resectable GIST.