9009 Background: Imatinib mesylate benefits over 80% of patients with metastatic GIST, but whether it is tolerable and effective as an adjuvant therapy after complete resection of primary GIST is unknown. Methods: Patients who had undergone complete gross resection of a primary GIST that expressed KIT and who were at high risk (tumor size ≥10 cm, tumor rupture, or <5 peritoneal metastases) for recurrence were accrued from 48 institutions. Prospective central pathologic confirmation was an inclusion criterion and the diagnosis could not be confirmed in 9% of patients referred for entry. All patients were prescribed imatinib 400 mg daily for 1 year. Results: Among the 106 evaluable patients, the median age was 58 (19–79) years, with 60 (57%) males. The median tumor size was 13 (range 3–42) cm and most tumors originated in the stomach (50%) or small intestine (42%). There were 89 (84%) patients who had a tumor ≥10 cm and 28 (26%) patients had tumor rupture or multifocal disease. Imatinib was started at a median of 59 (range 25–84) days after operation. All patients have been in the study for at least 1 year. No grade 4 or 5 toxicity occurred. There were 19 (17%) patients who had grade 3 toxicity, the most common being neutropenia (2%), dermatitis (2%), or increased ALT (2%). The most frequent toxicities of any grade were edema (55%), fatigue (43%), nausea (42%), diarrhea (42%), and dermatitis (27%). Dose reduction due to toxicity occurred in 23 (22%) patients. There were 87 (82%) patients who completed the 1 year of imatinib therapy: 72 (68%) patients tolerated full dose imatinib without dose modification, 8 had a temporary dose reduction, and 7 finished at a lower dose. There were 19 (18%) patients who did not complete therapy, including 6 who had toxicity and 12 who withdrew consent. Conclusions: Imatinib given at a daily oral dose of 400 mg is well tolerated by most patients following the resection of a high risk primary GIST. Further follow up is required to determine the effect of adjuvant imatinib on survival in high risk GIST. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Novartis Novartis Novartis Novartis