We studied the immunogenicity of trivalent-inactivated influenza vaccine. Subjects were 259 children under 4 years old who visited six pediatric clinics to undergo influenza vaccination. Age distribution was 64 aged <1.0, 65 aged 1.0-1.9, 64 aged 2.0-2.9, and 66 aged 3.0-3.9 years, including subjects who had been previously vaccinated within the last three years, 0% (0/64) aged <1.0, 26% (17/65) aged 1.0-1.9, 72% (46/64) aged 2.0-2.9, and 77% (51/66) aged 3.0-3.9 years old. Two doses of vaccine were given subcutaneously four weeks apart. Dosage was 0.l mL for children under 1 year old, while for children aged one year or older, dosage was 0.2mL, based on standard Japanese recommendations. To measure hemagglutination inhibition (HI) antibody titer, triplet sera were obtained before vaccination (S0), 4 weeks after the first vaccination (S1), and 4 weeks after the second vaccination (S2). The geometric mean of HI antibody titer, the response proportion (titer rise > or =4-fold), and the achievement proportion (postvaccination titer > or =1 : 40) were calculated by age group. Analysis of variance was used to estimate the independent effect of age and prevaccination titer on antibody increase. The geometric means of HI antibody titer were lower among the two younger age groups than among the two older age groups, regardless of vaccine strain or when blood samples were collected. The achievement proportion after 2 doses of vaccine in the <1.0, 1.0-1.9, 2.0-2.9, 3.0-3.9 year age groups were 38%, 58%, 89%, and 85% against A (HI) ; 52%, 54%, 81%, and 73% against A (H3) ; and 23%, 49%, 67%, and 71% against B. Regarding the analysis of variance, prevaccination titer consistently indicated strong effects on antibody increase, regardless of vaccine strain or combination of paired sera. After two doses of vaccine (S2/S0), significant effects of age on antibody induction were shown against A (H1) and B (p = 0.000 and 0.002). Thus, the immunogenicity of trivalent-inactivated influenza vaccine was strongly influenced by prevaccination titer and age. Even two doses of vaccine did not induce a protective antibody level in about 50 to 80% of subjects among infants aged <1.0 year, and 40 to 50% among children 1.0-1.9 year old.