Inclusion Criteria:

   - NOTE: the following eligibility criteria were applicable to acute lymphoblastic
   leukemia (ALL) and acute myelogenous leukemia (AML) patients for the Phase 1 portion
   of this study, and to ALL patients for the Phase 2 portion of the study (only ALL
   patients were allowed in the Phase 2 portion of the study).

   - ALL with > 25% blasts in bone marrow; AML with ≥ 5% blasts in bone marrow; ALL and AML
   patients may have extramedullary disease

   - Karnofsky Performance Status ≥ 50 for patients > 10 years old; Lansky Performance
   Status ≥ 50 for patients ≤ 10 years old

   - Prior therapy: AML: 1-2 prior induction regimens and ≤ 1 hematopoietic stem cell
   transplant (HSCT); ALL: 1-3 prior induction regimens

   - Adequate liver, renal, pancreatic, and cardiac function

   - Have received no prior HSCT (study amended in Phase 2 to exclude patients with prior
   HSCT)

Exclusion Criteria:

   - NOTE: the following eligibility criteria were applicable to ALL and AML patients for
   the Phase 1 portion of this study, and to ALL patients for the Phase 2 portion of the
   study (only ALL patients were allowed in the Phase 2 portion of the study).

   - Burkitt's leukemia

   - Previous treatment with clofarabine

   - Uncontrolled systemic fungal, bacterial or other infection and 48 hrs negative blood
   cultures required for patients with a history of fever within 3 days of enrollment

   - Active CNS involvement (i.e., should be CNS1 or CNS2)

   - Inadequate time since last therapy: ≤ 14 days since last cytotoxic chemotherapy; ≤ 7
   days since last biologic therapy; ≤ 14 days since last monoclonal antibody therapy

   - Have received prior HSCT (study amended in Phase 2 to exclude patients with prior
   HSCT)

   - Pregnant or lactating

   - Have tested positive for hepatitis B or hepatitis C infection or history of cirrhosis