Posaconazole Versus Micafungin for Prophylaxis Against Invasive Fungal Infections During Neutropenia in Patients Undergoing Chemotherapy for Acute Myelogenous Leukemia, Acute Lymphocytic Leukemia or Myelodysplastic Syndrome

Inclusion Criteria: * Subjects of greater than or equal to 18 years of age of either sex and of any race. Disease definition: * Anticipated or documented prolonged neutropenia (ANC\<500/mm3 \[0.5x109/L\]) at baseline or likely to develop within 3 to 5 days and lasting for at least 7 days due to: * Intensive induction chemotherapy for new diagnosis of acute myelogenous leukemia, acute lymphocytic leukemia or myelodysplastic syndrome receiving standard anthracycline based chemotherapy * Re-induction of acute myelogenous or lymphocytic leukemia after primary relapse * Myelodysplastic syndromes requiring induction (myelosuppressive) chemotherapy * Female subjects of childbearing potential must have a negative serum pregnancy test as per MSKCC guidelines. * Able to swallow oral medications Exclusion Criteria: * Subjects with history of presumed or proven invasive fungal infection within 30 days of randomization. * Subjects who are taking the following: Drugs known to interact with posaconazole and that may lead to life-threatening side effects (terfenadine, cisapride, and ebastine at entry or within 24 hours before entry, or astemizole at entry or within 10 days before entry); b. Drugs known to lower the serum concentration/efficacy of posaconazole: cimetidine, rifampin, carbamazepine, phenytoin, rifabutin, barbiturates, and isoniazid at entry or within 24 hours before entry; c. Subjects who are planned to receive \> 2mg flat dose of vinca alkaloids. * Subjects with a history of hypersensitivity or idiosyncratic reactions to azole agents. * Subjects with renal insufficiency (estimated creatinine clearance less than 20 mL/minute at baseline or likely to require dialysis during the study). * Subjects having an electrocardiogram with a prolonged QTc interval by manual reading: QTc greater than 490 msec. * Subjects with moderate or severe liver dysfunction at baseline, defined as aspartate aminotransferase, alanine aminotransferase, or alkaline phosphatase levels greater than 5 times the upper limit of normal (ULN), or a total bilirubin level greater than 3 times the ULN. * Subjects who are undergoing re-induction chemotherapy and have participated in this study during their first induction chemotherapy. * Subjects who will be receiving dasatinib.