Introduction: B-cell precursor Acute Lymphoblastic Leukemia (BCP-ALL) corresponds to about 80% of ALL, mainly affecting children and adolescents. Relapses affect about 20% of patients, with 40-70% overall survival after recurrence, highlighting the need for new therapeutic approaches. The CD105 molecule (endoglin) is a TGF-β co-receptor, its expression in BCP-ALL blast cells is related to a poor prognosis. In this context, the analysis of CD105 expression in normal and neoplastic lymphoid precursors can be an important tool to understand the role of this molecule. In addition, it can contribute to monitoring Measurable Residual Disease (MRD) analysis. Methods and Results: Bone marrow (BM) samples from patients diagnosed with BCP-ALL, aged between 0 and 18 years, treated at Hospital Aristides Maltez (HAM) were analyzed. Samples were previously evaluated by flow cytometry for diagnostic purposes (D0) and MRD follow-up (D33). Panels of antibodies that meet the classification of the World Health Organization (WHO) were used, with the inclusion of the anti-CD105 antibody. We evaluated CD105 expression in normal B cell precursors, mature B cells, and leukemic blasts from pediatric patients with BCP-ALL. CD105, at diagnosis, was expressed in 73.33% of patients with BCP-ALL, with the highest expression in leukemic blasts. When compared with DRM, the highest expression was in the hematogonia in negative DRM (67.28%), median fluorescence intensity (MFI) of 87.26. When analyzed individually (monitoring from D0 to relapse of one patient), the blasts had a higher expression and MFI, 21.56% and 26.14, respectively. Conclusion: We observed positive CD105 expression in 33% of the included patients, with higher population expression and high MFI. Furthermore, we observed a higher expression of CD105 in more immature B cells, suggesting a correlation with the maturation process of the B lineage.