The strategy for treating relapsed and refractory adult ALL patients is through reinduction chemotherapy followed by allogeneic stem cell transplantation, provided that the toxicity of the salvage regimen is acceptable. However, this leukemia is characterized as being highly refractory to standard chemotherapy and therefore novel therapeutic approaches are desperately needed. Clofarabine is a second generation nucleoside analog FDA approved for the treatment of relapsed and refractory pediatric ALL. Clofarabine has been administered to adult patients with hematologic malignancies with an acceptable toxicity profile with 8% of relapsed ALL patients attaining a complete response (CR). The maximum tolerated dose (MTD) of clofarabine IV in adult patients has been determined to be 40 mg/m2/day for 5 consecutive days, which is lower than the tolerable daily dose for pediatric patients, 52 mg/m2/day. More recently, Karp and colleagues reported their experience with clofarabine in combination with cyclophosphamide in 18 patients with refractory acute leukemias. Encouraging responses were seen in the refractory ALL patients with 67% (4/6) patients experiencing a CR. Toxicity did not allow dose escalation of clofarabine and the MTD was defined as 10 mg/m2 administered over 6 non-consecutive days when combined with cyclophosphamide 200-400mg/m2 over a total of 7 days per cycle. As such, we are conservatively evaluating a clofarabine dose of 20mg/m2 for five days with a dose de-escalation step if there is dose limiting toxicity.