This is an open-label study of MK0991 in children between 3 to 24 months of age with new onset fever and neutropenia. The purpose of the study is to investigate plasma drug levels of caspofungin.

Inclusion Criteria:

• Patient is 3 to 24 months of age with one or more of the following conditions:
• Leukemia, lymphoma, or other cancers
• Bone marrow or peripheral stem transplantation
• High dose chemotherapy leading to a decrease in white blood cells
• Aplastic anemia
• Patient has an absolute neutrophil count <500 mm3 (a specific type of white blood cell that fights infection) AND at least one recording of fever > 38 degreesC (oral or oral equivalent) within 72 hours of screening.

Exclusion Criteria:

• Patient is <3 months or >24 months of age at the time of study drug administration
• Patient has proven or probable invasive fungal infection at the time of enrollment
• Patient has certain blood clotting or liver function abnormalities
• Patient is hemodynamically unstable, exhibits hemodynamic compromise or is not expected to survive at least 5 days
• Patient is taking rifampin, cyclosporin A, phenytoin, carbamazepine, or phenobarbital.