Objectives: Many studies have demonstrated that microRNA-21 (miR-21) is an oncogene and is upregulated in tumor tissue. However, its association with B-cell acute lymphoblastic leukemia (B-ALL) remains poorly understood. Methods: The expression of miR-21 was detected by real-time quantitative PCR in 75 children with de novo B-ALL as well as in 50 healthy controls. This study was conducted to evaluate the miR-21 as a biomarker for risk assessment, diagnosis and prognosis. Results and discussion: Compared with normal controls, miR-21 expression was significantly upregulated in childhood B-ALL patients. Using the receiver operating characteristic curve 3.23 was selected as the cut-off value of miR-21 expression in distinguishing patients from controls. Patients group with High miR-21 expression was significantly associated with those aged <2 and >10 years, lower platelets count, more incidence of CNS infiltration and poorer treatment outcome also, they showed a significantly poorer disease-free survival (DFS) and overall survival (OS) compared to those with low miR-21 expression group. Its expression was an independent prognostic marker according to multivariate analysis. Conclusion: This is the first report demonstrating the upregulation of miR-21 in childhood BALL, and its association with poor response to induction therapy, shorter DFS and OS. These results suggest that miR-21 upregulation represent an unfavorable prognostic marker in Childhood B-ALL.