The purpose of this study is to determine whether a tablet form of azacitidine that taken by mouth is safe. This Phase I study will also look at different doses and different treatment schedules in order to better understand the effects (positive and negative) of oral azacitidine on the body and on the disease MDS, AML and CMML.

Optional Extension Phase (OEP) to the AZA PH US 2007 CL 005 study which allows subjects who continue to receive oral azacitidine and have stable disease or are demonstrating clinical benefit as assessed by the Investigator, and have consented to participate, may enter the OEP of this study (at their current doses) at the start of their next cycle.

Subjects who are entering the OEP should be discontinued from Part 1 or Part 2 protocol prescribed therapy in the AZA PH US 2007 CL 005 study.

Subjects may continue to receive oral azacitidine in the OEP until they meet the criteria for study discontinuation or oral azacitidine becomes commercially available. Subjects discontinuing from the OEP will have an OEP discontinuation visit 28 days after the last dose of study drug or at study withdrawal.

Primary Objective of OEP is to evaluate long term safety of oral azacitidine.

Inclusion Criteria:

• 18 years or older.
• Diagnosis of low or Int-1 risk MDS
• Low platelet count, and/or low hemoglobin, and/or RBC transfusion-dependent and/or platelet transfusion-dependent
• ECOG Performance status 0-2
• Standard safety inclusion for serum creatinine, AST, ALT, bilirubin.
• Serum bicarbonate greater than or equal to 20 mEq/L.
• Use of acceptable birth control.
• Signed, written informed consent.

Exclusion Criteria:

• Diagnosis of acute PML.
• Previous or concurrent malignancy.
• Prior treatment with azacitidine or other demethylating agents.
• Treatment with any anticancer therapy or investigational drugs within 21 days.
• Hypersensitivity to azacitidine or mannitol.
• Presence of GI disease.
• Active, uncontrolled infection.
• Known Human Immunodeficiency Virus (HIV) or Hepatitis C, or known active viral Hepatitis B.
• Breastfeeding or Pregnant females;
• Presence of serious illness, medical condition, or other medical history which would be likely to interfere with a subject's participation in the study or with the interpretation of the results.
• Current congestive heart failure (NY Heart Association Class III-IV), unstable angina or angina requiring surgical or medical intervention within 6 months, myocardial infarct within 6 months, or uncontrolled cardiac arrhythmia.