We evaluated infant sera from an immunization trial in Bangladesh to assess influenza hemagglutination inhibition antibody titer increases in 131 unimmunized infants from birth to 6 months. We detected 31 serologically defined infections. Combined with 10 additional rapid test-proven influenza cases, the minimal estimated incidence was 31 of 100 infants (95% CI: 24-41). These data suggest a high burden of influenza in young infants in tropical South Asia.