Background: Despite improved outcomes in pediatric acute leukemia, those who relapse or are refractory do poorly. Mitoxantrone and clofarabine as single agents have proven efficacy in pediatric leukemia. Our previously reported phase I results demonstrated the MTD of this combination to be clofarabine 35 mg/m² x 5 days and mitoxantrone 12 mg/m² x 4 days. Here, we present our phase II data utilizing the RP2D. Methods: Prospective, open label safety and efficacy study (NCT01842672). Patients 0-30.99 yr old with ALL or AML with relapse OR induction failure were given 1 to 3 cycles of clofarabine (RP2D 35 mg/m2/day) Day 1-5, in combination with mitoxantrone 12 mg/m2/day on Day 3-6. Dexrazoxane was given prior to mitoxantrone. CNS prophylaxis was achieved with intrathecal liposomal cytarabine. MRD was defined by flow cytometry (≥ 0.01%). Results: Total 21 patients have enrolled at the RP2D clofarabine. Median age 8 yrs (0.5-22 yrs). Patient characteristics include 13 ALL (8=Induction Failure, 4=Relapse 1, 1=Relapse 2) and 8 AML (4=Induction Failure, 4=Relapse 1). Overall regimen was well tolerated. One patient with grade IV prolonged myelosuppression. Median time to neutrophil recovery was 24 days. Eighteen of 21 (85.7%) patients achieved an MRD negative CR. Three patients died of progressive disease. All 18 patients achieving CR went on to receive an allogeneic HSCT with no documented transplant associated toxicities attributed to prior MITCL therapy. One patient developed MRD (2.6%) at Day +270 post HSCT and underwent subsequent therapy. The remaining 17 patients continue to demonstrate MRD negativity. Two patients died of transplant associated complications. The overall and event free survival for patients who responded to therapy are 89% (CI₉₅ 0.64-0.98) and 83% (CI₉₅ 0.58-0.96), respectively at a median follow up time of 24 months (range 2-58). Conclusions: The combination of clofarabine and mitoxantrone reinduction therapy for relapsed or refractory acute leukemia has been demonstrated to be safe and well tolerated at a RP2D of 35 mg/m² clofarabine in children with poor risk acute leukemias. Phase II data is encouraging with 85.7% MRD negative CR rate in leukemic patients allowing patients to safely proceed to AlloHSCT with a 2 year survival rate of 89%. Longer follow up and a larger cohort is planned and ongoing. Clinical trial information: NCT01842672