The primary purpose of this study is to identify the optimal dose of voriconazole, an anti-fungal drug often used in people undergoing stem cell transplant. An optimal dose level is one level that provides a good blood level (concentration) of voriconazole without too much toxicity.

Inclusion Criteria:

• Any patient undergoing allogeneic hematopoietic stem cell transplantation (either 1st or subsequent)
• Age ≤ 21 years
• Adequate organ function within 14 days of enrollment, i.e. Creatinine: < 1.5 x ULN and Hepatic: ALT, AST and total bilirubin < 3 x ULN
• Requires voriconazole to prevent or treat invasive fungal infection after undergoing stem cell transplantation

Exclusion Criteria:

• Has received voriconazole within 5 days prior to starting study therapy
• History of hypersensitivity or severe intolerance to azoles
• History, or current evidence, of cardiac arrhythmias defined as QTc ≥ 480 mm/sec
• Receiving the following drugs and cannot be discontinued at least 24 hours before starting therapy: pimozide, quinidine, astemizole, ergot alkaloids.
• Received one or more of the following drugs within 14 days prior to starting study, as they are potent inducers of hepatic microsomal enzymes: rifampin, rifabutin, carbamazepine, phenytoin, nevirapine, long-acting barbiturates.
• Received sirolimus within the 14 days prior to starting study as voriconazole is a potent inhibitor of sirolimus metabolism
• Receiving or anticipated need for methadone as co-administration with voriconazole potentially increases methadone exposure