OBJECTIVE: To compare the persistence, susceptibility and resistance of influenza A and influenza B viruses in oseltamivir therapy in outpatients of various ages. METHODS: Virus isolation was done before and 4-6 days after the initiation of oseltamivir therapy for 148 patients with influenza A/H3N2 and for 66 with influenza B in the 2003-2004 and 2004-2005 influenza seasons. Neuraminidase inhibition assay and neuraminidase or hemagglutinin sequence analysis were done using influenza viruses isolated from these patients. RESULTS: The virus isolation rate after oseltamivir therapy was significantly higher for influenza B (33.3%) than for influenza A/H3N2 (12.8%, p<0.001). The mean IC(50) values before and after oseltamivir therapy were significantly higher in patients with influenza B (10.82 and 11.32nM, respectively) than in patients with influenza A/H3N2 (0.94 and 0.81nM, respectively, both p<0.001). No significant differences in IC(50) among each age group, or no significant increase in IC(50) from before to after oseltamivir therapy was observed. Neuraminidase or hemagglutinin sequence analysis revealed no known genotype with resistance to oseltamivir. CONCLUSION: Virus persistence after oseltamivir therapy was longer and IC(50) values were higher in influenza B than influenza A.