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Purpose: In 2004, we started an intergroup randomized trial of adjuvant imatinib versus no further therapy after R0-R1 surgery patients with localized, high- or intermediate-risk GI stromal tumor (GIST)., Patients and Methods: Patients were randomly assigned to 2 years of imatinib 400 mg daily or no further therapy after surgery. The primary end point was overall survival; relapse-free survival (RFS), relapse-free interval, and toxicity were secondary end points. In 2009, given the concurrent improvement in prognosis of patients with advanced GIST, we changed the primary end point to imatinib failure-free survival (IFFS), with agreement of the independent data monitoring committee. We report on a planned interim analysis., Results: A total of 908 patients were randomly assigned between December 2004 and October 2008: 454 to imatinib and 454 to observation. Of these, 835 patients were eligible. With a median follow-up of 4.7 years, 5-year IFFS was 87% in the imatinib arm versus 84% in the control arm (hazard ratio, 0.79; 98.5% CI, 0.50 to 1.25; P = .21); RFS was 84% versus 66% at 3 years and 69% versus 63% at 5 years (log-rank P < .001); and 5-year overall survival was 100% versus 99%, respectively. Among 528 patients with high-risk GIST by local pathologist, 5-year IFFS was 79% versus 73%; among 336 centrally reviewed high-risk patients, it was 77% versus 73%, respectively., Conclusion: This study confirms that adjuvant imatinib has an overt impact on RFS. No significant difference in IFFS was observed, although in the high-risk subgroup there was a trend in favor of the adjuvant arm. IFFS was conceived as a potential end point in the adjuvant setting because it is sensitive to secondary resistance, which is the main adverse prognostic factor in patients with advanced GIST., (© 2015 by American Society of Clinical Oncology.)