This open-label, randomized, adaptive, 2-arm, multicenter study will evaluate the pharmacokinetics and pharmacodynamics of oseltamivir (Tamiflu) in immunocompromised children, less than (<) 13 years of age, with confirmed influenza infection. Participants will be randomized to receive either the standard dose or triple dose of oseltamivir orally daily for a minimum of 5 days and up to 20 days. Infants <1 year of age will be randomized to the standard dose arm only.