We report on the evaluation of the immunogenicity and reactogenicity/safety of AS03-adjuvanted vaccine against pandemic influenza A/H1N1/2009 in young children. In this open-label, randomized study, 157 healthy children aged 6-35 months received two doses (21 days apart) of split-virion inactivated A/California/7/2009 H1N1 vaccine containing either (i) 1.9microg hemagglutinin (HA) and AS03(B) (5.93mg tocopherol) (N=104) or (ii) 3.75mug HA and AS03(A) (11.86mg tocopherol) (N=53). At 21 days following the first dose of AS03(B)-adjuvanted vaccine (1.9microg HA) the percentage of children with hemagglutination-inhibition titers of >or=40 against the vaccine strain rose from 3.0% before vaccination to 100%. The seroconversion rate was 99% and the geometric mean titer (GMT) increased from 6 to 313. After the second dose the GMT increased further to 2008. The higher dose AS03(A)-adjuvanted 3.75microg HA vaccine did not further increase the immune response. Solicited symptoms reported within 7 days following vaccination were mainly mild to moderate. After the first dose of AS03(B)-adjuvanted vaccine (1.9microg HA) the most common solicited symptoms were pain at the injection site (35.6%) and irritability (31.7%). Fever (axillary >or=37.5 degrees C) was reported with an incidence of 20.2%. After the second dose reactogenicity tended to increase (injection site pain: 41.3%; irritability: 46.2%; fever >or=37.5 degrees C: 67.3%). Spontaneously reported adverse events with an intensity that prevented normal activities were documented for 2.9-6.7% of doses with only one event (vomiting) considered related to vaccination. There was one serious adverse event reported in the AS03(A)-adjuvanted 3.75microg HA vaccine group (traumatic brain injury) which was not considered as related to vaccination. In conclusion, these data suggest that a first dose of AS03(B)-adjuvanted A/H1N1/2009 vaccine containing 1.9microg HA in children 6-35 months old is highly immunogenic and that the overall reactogenicity profile is acceptable although reactions including fever tend to increase after a second dose.