Children with acute lymphoblastic leukemia (ALL) are at high risk of thrombotic complications, resulting from multiple risk factors (malignancy, chemotherapy, central venous access devices, and inherent host characteristics). Non-O blood groups have been associated with an increased risk of venous thromboembolism (VTE) in adults, with a compounding effect in the presence of thrombophilia or cancer. We hypothesized that among children with ALL receiving a standardized protocol, there would be an increased risk of thrombotic events in non-O compared with O blood group patients. In a retrospective study of 523 children with ALL from June 1995 to April 2013, there were 56 (10.7%) thromboembolic events. Patients with VTE were compared with the whole cohort, based on blood group, age, sex, leukemia phenotype, and clinical risk category. Among children with VTE, 42 (75%) had non-O and 14 (25%) had O blood group, compared with 302 (57.7%) non-O and 221 (42.3%) O blood groups in the cohort. Non-O blood group was confirmed as an independent risk factor for VTE in multivariate analysis. This is the first study to report a significant association between non-O blood groups and VTE in children with cancer.