Ph II Study of Azacitidine in Myelofibrosis

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: Hemoglobin (Hb) \< 10 g/dl, White Blood Cell (WBC) \< 4 or \> 30 x 10\*9/L; risk group: 0 = low, 1 = intermediate, 2 = high). * Performance 0-2 Eastern Cooperative Oncology Group (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 Upper Limit of of Normal(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.