The goal of this clinical research study is to learn if azacitidine can help to control MF. The safety of azacitidine in patients with MF will also be studied.

Optional Procedures: You will be asked to have additional blood samples and bone marrow samples collected. These samples will be used to evaluate important characteristics of MF before and during the therapy with azacitidine.

Azacitidine is a drug that is designed to block certain genes in cancer cells whose job is to stop the function of the tumor-fighting genes. By blocking the "bad" genes, the tumor-fighting genes may be able to work better.

Before you can start treatment on this study, you will have what are called "screening tests." These tests will help the doctor decide if you are eligible to take part in the study. You will have a bone marrow biopsy and aspirate to check on the status of the disease. To collect a bone marrow biopsy, an area of the hip or chest bone is numbed with anesthetic and a small amount of bone marrow and bone is withdrawn through a large needle. To collect a bone marrow aspirate, an area of the hip or chest bone is numbed with anesthetic and a small amount of bone marrow is withdrawn through a large needle. Blood (about 4 teaspoons) will be drawn for routine tests. Women who are able to have children must have a negative blood-pregnancy test.

If you are found to be eligible to take part in this study, you will be able to begin treatment with azacitidine. You will receive azacitidine as an injection under the skin once a day for 7 days in a row. This will be repeated every 4 weeks (4 weeks equals 1 cycle). The first cycle of azacitidine will be given at M. D. Anderson, in an outpatient setting. Later cycles of treatment courses may be given at M. D. Anderson or by a cancer doctor in your community.

You may receive up to 12 cycles of treatment if you are responding well to treatment. You will be taken off study if your disease gets worse or intolerable side effects occur. Once you go off study, you will receive follow-up as is standard of care for your disease.

This is an investigational study. Azacitidine is FDA approved for the treatment of myelodysplastic syndrome. Its use in this study is experimental.

Pharmion Corporation will provide you with azacitidine free of charge while you are being treated on this study. The optional procedures will be free of charge to participants. You or your insurance will be responsible for the costs of all other tests and procedures. A total of up to 34 patients will take part in this study. All will be enrolled at M. D. Anderson.

Optional Procedures: If you agree, between 4-10 teaspoons of blood will be drawn within 24 hours before starting therapy, once a week (+/- 2 days) during the first cycle of therapy, and then on Days 1 and 7 of each later cycle. The total amount of blood drawn for these optional tests will depend on the number of cycles you receive.

If you agree, you will also have an additional bone marrow biopsy collected once every 2 cycles.

You do not have to agree to take part in the optional procedures in order to receive treatment on this study.

Inclusion Criteria:

• Diagnosis of MF requiring therapy, including those previously treated and relapsed or refractory, or if newly diagnosed, with intermediate or high risk according to Lille scoring system (adverse prognostic factors are: Hb < 10 g/dl, WBC < 4 or > 30 x 10*9/L; risk group: 0 = low, 1 = intermediate, 2 = high).
• Performance 0-2 (ECOG).
• Signed informed consent.
• Patients must have been off chemotherapy for 2 weeks prior to entering this study and have recovered from the toxic effects (grade 0-1) of that therapy. Patients are allowed to be on anagrelide and hydroxyurea to control high platelet and WBC counts for their safety.
• Serum bilirubin levels </= 1.5 times the upper limit of the normal range for the laboratory (ULN). Higher levels are acceptable if these can be attributed to active hemolysis or ineffective erythropoiesis.
• Serum glutamic-oxaloacetic transaminase (SGOT) (aspartate aminotransferase [AST]) or serum glutamic-pyruvic transaminase (SGPT) (alanine aminotransferase [ALT]) levels </= 2 x ULN.
• Serum creatinine levels </= 1.5 x ULN; unless related to the MF, as judged by treating physicians.
• Women of childbearing potential must have a negative serum pregnancy test prior to azacitidine treatment and should be advised to avoid becoming pregnant. Men must be advised to not father a child while receiving treatment with azacitidine. Both women of childbearing potential and men must practice effective methods of contraception (those generally accepted as standard of care measures).
• Age >/= 18.

Exclusion Criteria:

• Nursing and pregnant females. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.
• Uncontrolled intercurrent illness including, but not limited to, uncontrolled active infection, symptomatic congestive heart failure, unstable angina pectoris, or psychiatric illness/social situations that would limit compliance with study requirements.
• Known or suspected hypersensitivity to azacitidine or mannitol.