Inclusion Criteria:

1. Patients must have relapsed/refractory leukemias for which no standard therapies are
anticipated to result in a durable remission. Patients with poor-risk myelodysplasia
(MDS) [i.e. refractory anemia with excess blasts (RAEB-1 or RAEB-2) by WHO
classification] and chronic myelomonocytic leukemia (CMML) are also candidates for
this protocol. Relapsed/refractory leukemias include acute non-lymphocytic leukemia
(AML) by WHO classification, acute lymphocytic leukemia (ALL), chronic lymphocytic
leukemia (CLL), or chronic myelogenous leukemia (CML) in blast crisis.

2. Patients with refractory/relapsed leukemia 16 years or older are eligible. Patients
60 years or older with newly diagnosed AML are eligible if they are not candidates
for, or if they refuse, intensive chemotherapy.

3. Eastern Cooperative Oncology Group (ECOG) performance status of 0-3.

4. Women of child-bearing potential (ie, a woman who has not been postmenopausal for at
least 12 consecutive months or who had not undergone previous surgical sterilization)
must use acceptable contraceptive methods (abstinence, intrauterine device [IUD],
oral contraceptive or double barrier device), and must have a negative urine
pregnancy test within 2 weeks prior to beginning treatment on this trial. Nursing
patients are excluded. Sexually active men must also use acceptable contraceptive
methods for the duration of time on study.

5. Continued from #4: Pregnant and nursing patients are excluded because the effects of
4'-thio-araC on a fetus or nursing child are unknown.

6. Must be able and willing to give written informed consent

7. In the absence of rapidly progressing disease, the interval from prior treatment to
time of study drug administration should be at least 2 weeks for cytotoxic agents or
at least 5 half-lives for noncytotoxic agents. If the patient is on hydroxyurea to
control peripheral blood leukemic cell counts, the patient must be off hydroxyurea
for at least 24 hours before initiation of treatment on this protocol. Persistent
clinically significant toxicities from prior chemotherapy must not be greater than
grade 1.

8. Patients must have the following clinical laboratory values unless considered due to
leukemic organ involvement: 1. Serum creatinine > 40 ml/min. 2. Total bilirubin due to Gilbert's syndrome. 3. Alanine aminotransferase (ALT), or aspartate
aminotransferase (AST) organ leukemic involvement.

9. Patients with active central nervous system (CNS) involvement of leukemia disease are
included and will be treated concurrently with intrathecal therapy.

Exclusion Criteria:

1. Uncontrolled intercurrent illness including, but not limited to uncontrolled
infection (e.g. requiring IV antibiotics, etc), symptomatic congestive heart failure,
cardiac arrhythmia, or psychiatric illness/social situations that would limit
compliance with study requirements.

2. Active heart disease including myocardial infarction within previous 3 months,
symptomatic coronary artery disease, QTc > 480, arrhythmias not controlled by
medication, or uncontrolled congestive heart failure defined as Class II to IV per
New York Heart Association Classification.

3. Patients receiving any other standard or investigational treatment for their
hematologic malignancy.

4. Patients with known HIV positive disease; patients with active hepatitis.