In this study we will measure the concentration of the drug called voriconazole which is used to fight infections caused by fungus in children who usually are cancer patients and have their immune system down. Since we know the dose in adults, and we think we know the matching doses in the young patients ages 2 to less than 15 years old, we will compare the amount of drug that goes into the system with what we know works in adults. We give the drug by a needle directly into the blood, then few days later we stop that and give the drug by mouth. Meanwhile, we draw a little bit of blood at certain times to measure the drug in it.