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