Primary Objective: To evaluate the efficacy of 3 different schedules of low-dose decitabine in the treatment of MDS in relation to response rates.

Secondary Objective: To evaluate response duration and survival with the 3 schedules. To evaluate the side effects with the 3 schedules.

Treatment: Methylation is a change that occurs to DNA that has an effect on gene usage in human cells. Abnormal methylation is very common in leukemias. Decitabine is a new drug that blocks DNA methylation.

Before treatment starts, a physical exam, blood tests (between 4-6 tablespoons), and a bone marrow study will be done. To collect a bone marrow sample, an area of the hip or chest bone is numbed with anesthetic and a small amount of bone marrow is withdrawn through a large needle. Women able to have children must have a negative blood or urine pregnancy test.

When this study began, participants were randomly assigned (as in the toss of a coin) to one of 3 treatment groups. The assignment to one of the 3 schedules was adjusted according to how well patients respond to treatment. About 17 patients were assigned to each group for the first 50 patients.

Participants in the first group received decitabine intravenously (IV--through a needle in their vein) over one hour, once a day, for 10 days. Treatment was given every 4 to 8 weeks depending on how well their blood counts recovered. Participants in the second group received decitabine as an IV infusion over one hour, once a day, for 5 days. Treatment was given every 4 to 8 weeks. Participants who received decitabine by vein got the same total dose per course. Participants in the third group received decitabine by subcutaneous (SQ) injections (injections given under the skin) twice a day for 5 days. As in the first and second group, treatment was given every 4 to 8 weeks.

After 65 patients were enrolled on this study, it was decided that the 5-day IV schedule was the best of the 3 schedules. The study will now continue with all new patients receiving the 5 -day IV decitabine treatment. If you are now enrolling on the study, you will be placed in this treatment group, instead of being randomly assigned to a treatment group.

Participants who are already on study and who are receiving the 5-day SQ schedule or the 10-day IV schedule, will be given the option to change to the 5-day IV schedule at the start of their next course of study drug treatment, since this is considered the new "standard" schedule on this particular study.

If you choose to take part in this study and begin receiving the study treatment described above, your response to treatment will be checked after completing 8 weeks of therapy. If the response to treatment is good, treatment with decitabine will continue. Decitabine treatment may be continued for up to 24 courses, or as long as it is judged best to control the leukemia.

During this study, you will need to visit your doctor for a physical exam and vital signs. The frequency of doctor visits will vary depending on your physical condition, but will be required at least once a month.

Blood tests (about 2 teaspoons) will be done about every week during the first 6-8 weeks of treatment, then every 1 to 2 weeks for the length of the study. The blood samples will be used for routine lab tests. Periodic bone marrow samples will also be taken to check cells related to the disease before, during, and after completion of this study.

Patients will be taken off study if the disease gets worse or intolerable side effects occur.

This is an investigational study. Decitabine is not yet FDA approved.Up to 133 participants will be treated in this study. All will be enrolled at M. D. Anderson.

• Myelodysplastic Syndrome
• Chronic Myelomonocytic Leukemia

• Active Comparator: Decitabine by vein for 10 Days
• Active Comparator: Decitabine by vein for 5 Days
• Active Comparator: Decitabine by injection under the skin for 5 days

Inclusion Criteria:

1. MDS and 5% or more marrow blasts, or IPSS risk intermediate 1-2 or high risk; or chronic myelomonocytic leukemia
2. Performance status 0-2 (ECOG scale); adequate hepatic (bilirubin < 2 mg/dl) and renal functions (creatinine <2mg/dl); NYHA cardiac status III-IV excluded.
3. Signed informed consent
4. No prior intensive combination chemotherapy or high-dose ara-C (>/= 1g/m2 per dose). Prior biologic therapies, targeted therapies and single agent chemotherapy allowed.
5. Patients must have been off chemotherapy for 2 weeks prior to entering this study and recovered from the toxic effects of that therapy, unless there is evidence of rapidly progressive disease. Use of Hydroxyurea for patients with rapidly proliferative disease is allowed for the first two weeks on therapy.

Exclusion Criteria:

1. Nursing and pregnant females are excluded. Patients of childbearing potential should practice effective methods of contraception. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.
2. Patients with active and uncontrolled infections
3. Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, or psychiatric illness/social situations that would limit compliance with study requirements