This study will assess the pharmacokinetics/pharmacodynamics and safety of oseltamivir [Tamiflu] therapy in infants less than 1 year of age with influenza diagnosed in the 96 hours prior to the first dose. Patients age 3-12 months will receive 3 mg/kg, 1-3 months will receive 2.5 mg/kg, and birth to 1 month will receive 2 mg/kg twice a day for a total of 10 doses. Patients positive for influenza virus on Day 6 will be eligible to receive continued study treatment for an additional 10 doses (5 days). The anticipated time on study treatment is 4 weeks, and the target sample size is 65-85 male and female infants.