10014 Background: Minimal (measurable) residual disease (MRD) at end of induction (EOI) therapy is a strong predictor of outcome in pediatric B-ALL. Currently, EOI MRD is assessed in bone marrow (BM). We hypothesized that the highly sensitive assay, high-throughput sequencing (HTS) of immunoglobulin loci, can effectively monitor MRD in peripheral blood (PB) and may provide a less invasive way to track therapy response. Methods: We conducted HTS MRD on paired EOI BM and PB samples from 808 NCI standard risk (SR) pediatric B-ALL patients enrolled on Children’s Oncology Group study AALL1731 (NCT03914625). We determined the correlation between BM and PB HTS MRD via Spearman’s rank correlations. We calculated the BM/PB MRD ratio and compared these by subgroup using Kruskal-Wallis tests. We defined subgroups by cytogenetics (cyto) ( ETV6::RUNX1, double trisomies of chromosome 4 and 10 (DT), Unfavorable (hypodiploidy, iAMP21, or KMT2A-rearranged), or Neutral (lacking ETV6::RUNX1, DT, or unfavorable)), and risk group (SR-average (AVG) and SR-High). Flow cytometry-defined EOI BM MRD was < 0.01% for all SR-AVG patients (N = 623) and ≥0.01% for selected SR-High patients (N = 185). Results: There was strong correlation between PB and BM HTS MRD with an overall correlation coefficient of 0.75 (P < 0.001). Correlation was similar by cytogenetics: ETV6::RUNX1, 0.69 (N = 63; P < 0.001), DT, 0.75 (N = 147; P < 0.001), Neutral, 0.74 (N = 580; P < 0.001), and Unfavorable, 0.66(N = 18; P = 0.003). For risk groups, correlation for SR-AVG was 0.67 (p < 0.001) and SR-High, 0.64 (p < 0.001). Of the 591 SR-AVG patients with detectable BM HTS MRD, PB HTS MRD was detectable in 474 (80.2%), undetectable in 94 (15.9%) and indeterminate (no leukemic cell detected and < 500,000 total cells in sample) in 23 (3.9%). Among 182 SR-High patients with detectable BM HTS MRD, 175 (96.2%) had detectable PB HTS MRD. Disease burden was higher in the BM than PB with a significantly higher BM/PB ratio in SR-High compared to SR-AVG patients (median 16.5 vs 2.6, P < 0.001). The median BM/PB ratio also varied by cytogenetics with those with Unfavorable cyto having the highest ratio (15.3 vs 6.3 in DT, 3.8 in ETV6::RUNX1, 3.1 in Neutral; P = 0.013). Conclusions: This is the largest analysis of paired B-ALL BM/PB HTS MRD to date. We show strong correlation between PB and BM across risk and cytogenetic groups. The ratio of BM/PB MRD varied and was highest among patients with Unfavorable cyto suggesting BM tropism. Importantly, PB MRD was detectable in nearly all patients with flow EOI BM MRD ≥0.01%, a threshold warranting therapy intensification. However, most patients with EOI BM flow MRD < 0.01% also had detectable PB HTS MRD. Thus, PB HTS MRD may provide a useful adjunct for screening and clinical management of B-ALL patients. Defining a PB HTS MRD threshold useful for risk stratification will require correlation with outcome.