 In this study, patients who have been diagnosed with gastrointestinal stromal tumor GIST will be randomly allocated in a 1:1 ratio to receive imatinib Gleevec either for 12 or for 36 months following surgery. The study participants are required to have a histologically verified GIST with a high risk of GIST recurrence despite complete removal of all macroscopic GIST tissue at surgery. The highvery high risk of recurrence is defined as one of the following: 1 the largest tumor diameter is over 10 cm; 2 the mitosis count is high over 10 mitoses per 50 high power microscope fields, HPFs; 3 the largest tumor diameter over 5 cm and the mitosis count is over 550 HPFs; 4 tumor spillage has taken place into the abdominal cavity at the time of surgery or following spontaneous tumor rupture. All study participants will receive imatinib 400 mgday orally, but the duration of imatinib administration will be determined randomly either for 12 or for 36 months. The study participants will be followed up using blood tests and computed tomography or MRI of the abdomen. The computed tomography examinations will be performed at 6 month intervals for a median of 5 years. A total of 280 patients will be entered into the study. The study hypothesis is that adjuvant imatinib may prevent some of the GIST recurrences, and that there may be a difference in the rate of GIST recurrence between the two groups. 