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.