Purpose: To analyze the prognostic impact of mutated KIT (mutKIT) in core-binding factor acute myeloid leukemia (AML) with inv(16)(p13q22) and t(8;21)(q22;q22). Patients and Methods: Sixty-one adults with inv(16) and 49 adults with t(8;21), assigned to postremission therapy with repetitive cycles of higher dose cytarabine were analyzed for mutKIT in exon 17 (mutKIT17) and 8 (mutKIT78) by denaturing high-performance liquid chromatography and direct sequencing at diagnosis. The median follow-up was 5.3 years. Results: Among patients with inv(16), 29.5% had mutKIT (16% with mutKIT17 and 13% with sole mutKIT8). Among patients with t(8;21), 22% had mut/C/T(18% with mutKIT17 and 4% with sole mutKIT8). Complete remission rates of patients with mutKIT and wild-type KIT (wtKIT) were similar in both cytogenetic groups. In inv(16), the cumulative incidence of relapse (CIR) was higher for patients with mutKIT(P = .05; 5-year CIR, 56% v 29%) and those with mutKIT17(P = .002; 5-year CIR, 80% v 29%) compared with wtKIT patients. Once data were adjusted for sex, mutKIT predicted worse overall survival (OS). Int(8;21), mutKIT predicted higher CIR (P = .017; 5-year CIR, 70% v 36%), but did not influence OS. Conclusion: We report for the first time that mutKIT, and particularly mutKIT17, confer higher relapse risk, and both mutKIT17 and mutKIT8 appear to adversely affect OS in AML with inv(16). We also confirm the adverse impact of mutKIT on relapse risk in t(8;21) AML. We suggest that patients with core-binding factor AML should be screened for mutKIT at diagnosis for both prognostic and therapeutic purposes, given that activated KIT potentially can be targeted with novel tyrosine kinase inhibitors. © 2006 by American Society of Clinical Oncology.