 This is an open-label, randomized, prospective, phase III, multicenter study carried out in the Nordic countries and in Germany. Following macroscopically complete surgery, the study participants will be allocated to receive imatinib either for 12 or for 36 months. At randomization, the patients are stratified into 2 strata: 1 local disease 1 GIST tumor; 2 intra-abdominal implants or resectable intra-abdominalhepatic metastases, or intra-abdominal spillage is present, or R1 surgery has been carried out microscopic disease has been left behind. The imatinib dose is 400 mgday administered with food. Imatinib dose adjustments are made as per protocol. 