Background: Survival after relapse in pediatric acute myeloid leukemia (AML) remains poor, highlighting the critical importance of identifying prognostic factors to guide optimal relapse management.

Methods: We investigated the prognostic impact of multiparameter flow cytometry (MFC) measurable residual disease (MRD) in 188 patients with first relapse after initial treatment according to the NOPHO-DBH AML 2012 protocol.

Results: The 4-year overall survival (OS_4y) was 44% (95% confidence interval [CI]: 36%–51%). OS_4y was 61% (CI: 51%–69%) in 133/188 patients who received stem cell transplantation (SCT) after reinduction therapy. Nineteen patients treated at the time of molecular relapse showed an excellent OS_4y of 84% (CI: 58%–95%). Patients with hematological relapse and MRD <0.1% after first reinduction course had a superior OS_4y of 69% (CI: 51%–81%) compared to patients with MRD between 0.1% and 4.9% (OS_4y 46%, CI:28%–63%) and MRD ≥5% (OS_4y 16%, CI: 6%–30%), adjusted hazard ratio (HR) 2.2 for MRD 0.1%–4.9%; p = 0.04 and HR 6.0 for MRD ≥5%; p < 0.001. Patients in second complete remission after first reinduction course and MRD <0.1% after second reinduction course had an OS_4y of 70% (CI: 52%–82%) compared to 46% (CI: 19%–70%) in patients with MRD ≥0.1%, HR 2.7; p = 0.048. OS_4y was 71% (CI: 54%–82%) and 31% (CI: 13%–51%) for patients with MRD <0.1% or ≥0.1% prior to SCT, respectively, HR 3.1; p = 0.004.

Conclusions: This study identifies MFC MRD during reinduction therapy and before SCT as novel and independent predictors of outcome in relapsed pediatric AML.