Internal tandem duplication (ITD) of the fms‐related tyrosine kinase‐3 gene (FLT3) confer a poor prognosis in adult AML. Studies have reported that a higher mutant allelic burden is associated with a worse prognosis. Adult patients with FLT3‐ITD mutated AML treated at our institution were identified. Patients were assigned into 2 groups; patients who received idarubicin and cytarabine (IA, group one) containing induction, and who received sorafenib in addition to IA containing regimens at induction (group two). The optimal FLT3‐ITD mutant allele cut‐off was defined as the cut‐off to divide the whole cohort with the highest statistical significance. A total of 183 patients including 104 (57%) in group one and 79 (43%) in group two were identified. The complete remission (CR)/CR with incomplete hematologic recovery (CRi) for group one and group two were 85% and 99%, respectively (P = .004). The median relapse free survival (RFS) for group one and two were 12 and 45 months, respectively (P = .02). The median overall survival (mOS) was 17 months in group one, and has not been reached in group two (P = .008). The optimal FLT3‐ITD mutant allele cut‐off for OS was 6.9% in group one, there was no optimal cut‐off in group two. On multivariate analysis, poor performance status (PS) (P = .003), sorafenib (P = .01), and presenting white blood cells (WBC) (P < .001) were independent predictors of OS. Higher FLT3‐ITD allele burden is associated with a worse outcome in patients treated with IA‐based chemotherapy. Addition of sorafenib to chemotherapy not only nullifies the negative prognostic impact of higher allele burden, but also improves outcome of FLT3‐ITD mutated AML patients regardless of the allele burden.