Inclusion Criteria:

1. Patients will be 16 years of age or older.
2. Patients must have relapsed/refractory leukemias for which no standard therapies are anticipated to result in a durable remission (longer than 3 months). Patients with poor-risk myelodysplasia (MDS) [i.e. refractory anemia with excess blasts (RAEB-1 or RAEB-2) by WHO classification (see Appendix G)] and chronic myelomonocytic leukemia (CMML) are also candidates for this protocol. Relapsed/refractory leukemias include acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), or chronic myelogenous leukemia (CML) in blastic phase.
3. Continued from #2: Elderly patients with AML who are not eligible for frontline standard therapy, or who refuse to be treated with intensive chemotherapy, may be eligible. The phase II portion of the study will enroll patients with AML, MDS, and ALL. Patients with CML and CMML will not participate in the phase II portion of the study. Patients who are being considered for stem cell transplant are also eligible for this protocol.
4. ECOG performance status of 0-3.
5. Women of child-bearing potential (i.e., woman has not been naturally postmenopausal for at least 24 consecutive months or not surgically sterile) must use acceptable contraceptive methods (abstinence, intrauterine device [IUD], oral contraceptive or double barrier device), and must have a negative serum or urine pregnancy test within 2 weeks prior to beginning treatment on this trial. Sexually active men must also use acceptable contraceptive methods for the duration of time on study. Men and women must maintain effective contraception until 4 weeks after the last dose of drug is administered.
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 (any grade 2 or worse toxicities, non-hematologic or hematologic) 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 </= 2.0 mg/dl; 2) Total bilirubin </= 1.5x the upper limit of normal unless considered due to Gilbert's syndrome; 3) Alanine aminotransferase (ALT), or aspartate aminotransferase (AST) </= 3x the upper limit of normal unless considered due to organ leukemic involvement.
9. Patients with active CNS disease are included and will be treated concurrently with intrathecal therapy.
10. Phase II Portion: All above criteria apply. After the phase I portion, patients eligibility will be for only 3 disease categories which will accrue in parallel: 1) AML, 2) MDS, and 3) ALL.

Exclusion Criteria:

1. Uncontrolled intercurrent illness including, but not limited to uncontrolled infection (i.e. persistent fever, clinical deterioration), acute congestive heart failure and exacerbation, cardiac arrhythmia, chronic liver disease, or psychiatric illness/social situations that would limit compliance with study requirements.
2. Active heart disease including myocardial infarction within previous 3 months, unstable angina, arrhythmias not controlled by medication, or uncontrolled congestive heart failure. Patients with NYHA class 3 or 4 are excluded.
3. Patients receiving any other standard or investigational treatment for their hematologic malignancy, except as permitted under Inclusion #9 above.
4. Pregnant or breast feeding females are excluded because the effects of bendamustine on a fetus or nursing child are unknown.