9514 Background: Infants (<366 days) with ALL and an MLL rearrangement (MLL-R), have a dismal prognosis. Early relapse is common and event free survival (EFS) is poor. The Children's Oncology Group (COG) parallel pilot trials, 1953 and 9407, were designed to prospectively compare early HSCT to intensified chemotherapy in infants with MLL-R ALL. This is the largest prospective trial investigating the role of HSCT in such patients (pts) in first remission (CR-1). Methods: From 1997–2000, 186 pts were registered (132 MLL-R). Both trials were identical through Induction, Induction Intensification and Re-Induction (Re-I), then diverged. Pts with MLL-R ALL with matched/one antigen mismatched - related/unrelated donors were to undergo HSCT (mandated on 1953/optional on 9407) at completion of Re-I. The HSCT regimen was ara-c, cyclophosphamide, steroid and total body irradiation; graft vs. host prophylaxis was cyclosporine. Results: Compliance with protocol mandated conditioning for HSCT was poor and 28/53 HSCT pt...