This experimental (investigational) study is intended to answer the question of whether decitabine is any better than supportive care alone in delaying progression (worsening) of the disease, prolonging survival or improving the overall quality of life for MDS patients who are not candidates for bone marrow transplant (BMT).

Inclusion:

• MDS (de novo or secondary) fitting any of the recognized French-American-British, FAB, classifications or chronic myelomonocytic leukemia (CMML) with WBC < 12,000/mm3, AND International Prognostic Scoring System (IPSS) >/= 0.5 as determined by CBC, bone marrow assessment and bone marrow cytogenetics within 30 days of randomization
• 18 years or older
• Female patients of child-bearing potential must have a negative serum hCG within 24 hours prior to randomization, must practice a medically approved method of birth control for the past 30 days, and agree to continue this practice for the trial duration and must not be breast-feeding
• ECOG or WHO performance status of 0-2
• Written informed consent
• Normal renal and hepatic function (creatinine </= 2 mg/dL, bilirubin </= 1.5 mg/dL, SGPT </= 2 times the upper limit of normal range)

Exclusion:

• Acute Myeloid Leukemia (AML) (>/=30% bone marrow blasts) or other progressive malignant disease
• Patients must have recovered from the toxic effects of prior therapy and must be off all chemotherapy for a minimum of 4 weeks prior to study entry to the protocol (a minimum of six weeks for prior nitrosoureas and bone marrow transplantation)
• Ongoing treatment with androgenic hormones, danazol, colony-stimulating factors, or other agents used to treat MDS within 7 days of study initiation.
• Administration of any investigational agent within the 30 days preceding study initiation.
• Uncontrolled cardiac disease or congestive heart failure
• Uncontrolled restrictive or obstructive pulmonary disease
• Active viral or bacterial infection
• Superimposed autoimmune hemolytic anemia or thrombocytopenia
• Known positive serology for HIV
• Mental illness or other condition preventing full cooperation with the treatment and monitoring requirements of the study.