BACKGROUND: A lower baseline bone marrow blast percentage (bBMB%) is associated with better outcomes in patients with B-cell acute lymphoblastic leukemia (B-ALL) receiving blinatumomab. The objective of this analysis was to investigate the association between bBMB% and treatment outcomes in relapsed/refractory (R/R) B-ALL. METHODS: Data from five trials of blinatumomab for R/R B-ALL were pooled for analyses. Patients were placed in one of three groups: group 1, ≥50% bBMBs; group 2, ≥25% to <50% bBMBs; group 3, ≥5% to <25% bBMBs. Response and survival outcomes were compared between groups. RESULTS: Data from 683 patients (166 pediatric, 517 adult) were analyzed. Collectively, patients in groups 2 and 3 had significantly higher odds of achieving a complete remission (CR) (odds ratio [OR], 3.50 [95% confidence interval (CI), 2.23-5.48] and 3.93 [95% CI, 2.50-6.18], respectively; p < .001) and minimal/measurable residual disease response (OR, 2.61 and 3.37, respectively; p < .001) when compared with group 1 (reference). Groups 2 and 3 had a 37% and 46% reduction in the risk of death (hazard ratio [HR], 0.63 and 0.54, respectively; p < .001) and a 41% and 43% reduction in the risk of an event (relapse or death) (HR, 0.59 and 0.57, respectively; p < .001) compared with group 1. No significant differences in response or survival outcomes were observed between groups 2 and 3. Seven of nine patients whose bBMB% was lowered to <50% with dexamethasone achieved CR with blinatumomab. CONCLUSION: Any bBMB% <50% was associated with improved efficacy following blinatumomab treatment for R/R B-ALL.