Patients undergoing treatment of acute lymphoblastic leukemia (ALL) are at risk for thrombosis, caused in part by the use of l-asparaginase (L-ASP). Antithrombin (AT) replacement has been suggested to prevent venous thromboembolism (VTE) and thus may increase exposure to ASP. We report herein the results of the prophylactic replacement strategy in the pediatrics-inspired prospective GRAALL-2005 study. Between 2006 and 2014, 784 adult patients with newly diagnosed Philadelphia- ALL were included. The incidence rate of VTE was 16%, with 69% of cases occurring during induction therapy. Most patients received AT supplementation (87%). After excluding patients who did not receive L-ASP or who developed thrombosis before L-ASP, AT supplementation did not have a significant impact on VTE. Administration of fibrinogen concentrates was associated with an increased risk of VTE, whereas transfusion of fresh frozen plasma had no effect. Heparin prophylaxis was associated with an increased risk of VTE. Prophylactic measures were not associated with an increased risk of grade 3 to 4 bleeding complications. The rate of VTE recurrence after L-ASP reintroduction was 3% (1 of 34). In ALL patients receiving L-ASP therapy, the use of fibrinogen concentrates may increase the risk of thrombosis and should be restricted to rare patients with hypofibrinogenemia-induced hemorrhage. VTE developed despite extensive AT supplementation, which suggests the need for additional prophylactic measures. Although this large descriptive study was not powered to demonstrate the efficacy of these prophylactic measures, it provides important insight to guide future trial design. This trial was registered at www.clinicaltrials.gov as #NCT00327678.