11503 Background: Adjuvant imatinib improves recurrence-free survival (RFS) when administered after surgery to selected patients with operable gastrointestinal stromal tumor (GIST). It is uncertain whether overall survival (OS) improves, since imatinib improved OS in only one of the 3 large randomized trials conducted, and in this trial (the Scandinavian Sarcoma Group XVIII/German trial; SSGXVIII/AIO; NCT00116935) the statistical significance for OS remained borderline. The objective of the present analysis was to evaluate long-term OS of patients who participated in the SSGXVIII/AIO trial. Methods: SSGXVIII/AIO is an open-label, randomized (1:1), multicenter phase 3 trial. Four hundred patients who underwent macroscopically complete surgery for GIST with a high estimated risk for recurrence according to the modified National Institutes of Health Consensus Criteria were accrued between February 2004 and September 2008. Imatinib was scheduled to be administered 400 mg/day orally for either 12 months or 36 months after surgery. The patients were scheduled to be followed up for 10 years after study entry. Imaging of the abdomen was carried out periodically. The primary end point was RFS; the secondary objectives included OS and treatment safety. Results: The median follow-up time was 119 months. In the Intention-To-Treat Population 194 RFS events and 96 OS events were recorded. In the 36-month group, 5-year and 10-year RFS was 71.4% and 52.5%, and in the 12-month group, 53.0% and 41.8%, respectively (HR 0.66, 95% CI 0.49-0.87; P = .003). In the 36-month group, the 5-year and 10-year OS rates were 92.0% and 79.0%, and in the 12-month group, 85.5% and 65.3%, respectively (HR 0.55, 95% CI 0.37-0.83; P = .004). In the Efficacy Population, from which 15 patients who did not have GIST in central pathology review and 24 patients who had intra-abdominal metastases removed at surgery were excluded, 10-year OS was 81.6% in the 36-month group and 66.8% in the 12-month group (HR, 0.50, 95% CI 0.32-0.80; P = .003). No new safety signals were detected. Conclusions: About 50% of deaths can be avoided during the first decade of follow-up after surgery with 3-year imatinib treatment as compared to 1-year treatment. Clinical trial information: NCT00116935 .