Inclusion Criteria:

          -  Age ≥ 18 years.

          -  Morphological and immunohistological documentation of GIST (immunostaining for KIT
             and/or DOG-1 positive, or mutation of KIT or PDGFRA present in tumor tissue).

          -  Macroscopically complete surgical resection of GIST (either R0 or R1 resection).

          -  Mutation analysis of KIT and PDGFR genes has been carried out.

          -  A high risk of GIST recurrence; either gastric GIST with mitotic count >10/50 HPFs, or
             non-gastric GIST with mitotic count >5/50 HPFs, or tumor rupture.

          -  Eastern Cooperative Oncology Group performance status ≤ 2.

          -  Adequate organ function.

          -  Female patients of childbearing potential must have a negative pregnancy test within
             14 days before initiation of study drug dosing. Postmenopausal women must have
             amenorrhea for at least 12 months to be considered of non-childbearing potential. Male
             and female patients of reproductive potential must agree to employ an effective
             barrier method of birth control throughout the study and for up to 3 months following
             discontinuation of study drug.

          -  Patient willing to be followed up at the study site regardless of the result of
             randomization.

          -  Patient has provided a written, voluntary informed consent prior to study-specific
             screening procedures.

        Exclusion Criteria:

          -  Presence of distant metastases or local recurrence of GIST.

          -  Not willing to donate tumor tissue and/or blood samples for the study molecular
             studies.

          -  Presence of a substitution mutation at PDGFRA codon D842 (usually D842V).

          -  Administration of adjuvant imatinib longer than for 3 years is planned regardless of
             the result of randomization, or "life long" imatinib administration is planned.

          -  Prior adjuvant (+ neoadjuvant) therapy with imatinib mesylate for at least 35 months
             has not been completed, or the total duration of prior adjuvant (+ neoadjuvant)
             imatinib administration exceeds the total duration of 37 months.

          -  Neoadjuvant imatinib for a duration that exceeds 9 months.

          -  Longer than 4-week break during adjuvant imatinib administration.

          -  The dose of imatinib at completion of 3 years of adjuvant imatinib was 200 mg per day
             or less or greater than 800 mg per day.

          -  Patient has received any investigational anti-cancer agents during adjuvant imatinib
             or between completion of adjuvant imatinib and the date of randomization.

          -  Patient has been free of another malignancy for less than 5 years except if the other
             malignancy is not currently clinically significant nor requiring active intervention,
             or if the other malignancy is a basal cell skin cancer or a cervical carcinoma in
             situ. Recent existence of any other malignant disease is not allowed.

          -  Patient with Grade III/IV cardiac disease as defined by the New York Heart Association
             Criteria (i.e., congestive heart failure, myocardial infarction within 6 months of
             study entry).

          -  Female patients who are pregnant or breast-feeding.

          -  Severe and/or uncontrolled medical disease (i.e., uncontrolled diabetes, severe
             chronic renal disease, or active uncontrolled infection).

          -  Known diagnosis of human immunodeficiency virus (HIV) infection.

          -  Patient with a significant history of non-compliance to medical regimens or with
             inability to grant reliable informed consent.