Background: Older adults with ALL have poor survival outcomes. Although aHSCT can be curative when used as consolidation after complete remission (CR), advanced age, limited performance status, and comorbidities are risk factors for increased non-relapse mortality (NRM) after myeloablative aHSCT. The 1-year disease free survival (DFS) for patients ≥ 40 years who receive an aHSCT for ALL is often estimated to be 40-50%. Previous studies have demonstrated the efficacy of TBI-based regimens in adults with ALL when combined with cyclophosphamide (Cy). Reduced intensity conditioning for ALL patients has fallen out of favor due to high relapse rate forfeiting the benefit of reduced NRM. High-dose Cy is, however, associated with cardiac, hemorrhagic and hepatic toxicities. Fludarabine (Flu) has emerged as a safer substitute of Cy (e.g. Flu/Bu replacing Bu/Cy) with favorable toxicity profile. Given the efficacy of TBI-based regimens in ALL, we hypothesized that a myeloablative regimen of Flu/TBI (12 Gy) is almost as effective as Cy/TBI 12 Gy in older adults with ALL undergoing aHSCT, but with less NRM confering survival benefit over Cy/TBI 12 Gy. Methods: This study is a single center, single-arm phase II clinical trial of Flu 40 mg/m² IV daily (days -7 to -4) and TBI 2 Gy X2 (days -3 to -1) as myeloablative conditioning for older adults (≥40 years old) or younger adults with significant comorbidities with ALL. Patients aged 40-65 years with ALL in CR, KPS ≥ 70, adequate organ function, and having HLA-matched sibling or unrelated donor will be eligible. The primary endpoint of the study is 1-year DFS post-transplant. Secondary endpoints include 1-year overall survival (OS), incidence and severity of acute and chronic GVHD, immune reconstitution and regimen related toxicity. The study has just finished accrual (January 2019) enrolling a total of 16 patients. This number of patients was pre-determined to give a probability ≤ 0.05 of concluding that the 1-year DFS rate exceeds 45%, and a probability of at least 0.80 of concluding that the 1-year DFS rate exceeds 45% (expecting 1-year DFS of 75%). Clinical trial information: NCT01991457