A Study of the Safety and Efficacy of Aprepitant for the Prevention of Chemotherapy-Induced Nausea and Vomiting (CINV) in Pediatric Participants (MK-0869-208)

Inclusion Criteria: Cycle 1: * Is 6 months to 17 years of age at time of study entry * Is scheduled to receive chemotherapeutic agent(s) associated with moderate, high risk or very high risk of vomiting for a documented malignancy, or a chemotherapy regimen not previously tolerated due to vomiting * Is expected to receive ondansetron as part of their antiemetic regimen * If female and has begun menses, must has a negative urine pregnancy test prior to randomization. A female who is of reproductive potential agrees to remain abstinent or use a barrier form of contraception for at least 14 days prior to, throughout, and for at least one month following the last dose of study medication * If \>10 years old, have a Karnofsky score ≥ 60; if ≤ 10 years have a Lansky Play Performance score ≥ 60 * Have a predicted life expectancy of ≥ 3 months Optional Cycles 2-6: * Participant has, in the opinion of the investigator, completed the preceding cycle of chemotherapy and related study procedures satisfactorily Exclusion Criteria: Cycle 1: * Has vomited in the 24 hours prior to Treatment Day 1 * Is scheduled to receive stem cell rescue therapy in conjunction with study related course(s) of emetogenic chemotherapy * Has received or will receive radiation therapy to the abdomen or pelvis within a week prior to Treatment Day 1 or during the course of the study * Is pregnant or breast feeding * Is allergic to aprepitant, ondansetron, or any other 5-hydroxytryptamine type-3 receptor (5-HT3) antagonist * Has a symptomatic primary or metastatic CNS malignancy causing nausea and/or vomiting * History of QT prolongation or taking other medicinal products that lead to QT prolongation * Has an active infection (e.g., pneumonia), congestive heart failure, bradyarrhythmia, or any uncontrolled disease (e.g., diabetic ketoacidosis, gastrointestinal obstruction) except for malignancy, which in the opinion of the investigator, might confound the results of the study or pose unwarranted risk in administering study drug to the participant * Has had benzodiazepine or opioid therapy initiated within 48 hours of study drug administration, except for single daily doses of triazolam, temazepam, or midazolam * Has been started on systemic corticosteroid therapy within 72 hours prior to study drug administration or is planned to receive a corticosteroid as part of the chemotherapy regimen * Is currently taking warfarin Optional Cycles 2-6: * All exclusion criteria from Cycle 1 apply except for vomiting in the 24 hours prior to Treatment Day 1