Inclusion Criteria:

• Histologically proven diagnosis of GIST, with positive immunostaining for KIT (CD117)
• Tumor size > 5 cm and mitotic rate > 5/50HPF, or tumor size > 10 cm irrespective of mitotic rate, or mitotic rate > 10/50 HPF irrespective of tumor size.
• Presence of mutation in exon 11 of c-kit gene.
• Surgery performed from 3 weeks to 8 weeks before administration of Imatinib mesylate.
• No evidence of residual macroscopic and microscopic disease after surgery.
• Absence of distant metastases
• No prior radiation therapy, no prior chemotherapy, no prior therapy with Imatinib mesylate, or any other molecular targeted or biological therapy.
• Age 18 yrs or older
• ECOG performance status = 0-2
• No New York Heart Association (NYHA) Class 3~4 cardiac problems
• Absence of severe and/or uncontrolled concurrent medical disease (e.g., uncontrolled diabetes, uncontrolled chronic renal disease, uncontrolled liver disease, including chronic viral hepatitis judged at risk of reactivation, uncontrolled active infection, such as human immunodeficiency virus (HIV) infection, etc.).
• No ongoing pregnancy or nursing..
• No prior, or ongoing other malignancy, except adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer or adequately treated cancer with eradicative intent for which the patient has been continuously disease-free for 5 years.
• No use of coumarine derivatives at the time of treatment start.
• Adequate liver function, as defined by a serum bilirubin < 1.5 x the institutional upper limit of normal (IULN), aspartate aminotransferase (AST) or alanine aminotransferase (ALT) < 2.5 IULN, obtained within 7 days prior to randomization.
• Adequate renal function, as defined by a serum creatinine < 1.5 x IULN, obtained within 7 days prior to randomization.
• Absolute neutrophil count (ANC) > 1.5 x 109/l and a platelet count > 100 x 109/l obtained within 7 days prior to randomization. Baseline hemoglobin > 9 g/dl (this may be achieved by transfusions if needed).
• Absence of any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be discussed with the patient before registration in the trial.