After the announcement of a worldwide pandemic in June 2009, a single dose of a monovalent pandemic H1N1 2009 influenza A (pH1N1/09) vaccine was advocated for all Australians who were 10 years and older because of excellent immunogenicity trial results for healthy children and adults. Immunocompromised patients have previously been shown to have lower seroconversion rates after routine vaccinations. There is a lack of data concerning the immune response of this patient group after pH1N1/09 vaccination. The aim of this study was to assess the immunogenicity of a pH1N1/09 vaccine in pediatric liver transplant recipients 10 years of age or older. Liver transplant recipients ≥ 10 years were prospectively recruited. All participants were administered a single intramuscular injection of the pH1N1/09 vaccine (15 μg). Serum antibody levels were determined by hemagglutination immediately before and ≥ 6 weeks after vaccination. Clinical and laboratory data (age, time since transplantation, immunosuppression, and lymphocyte counts) were analyzed comparing seroconverters and nonconverters with the Student's t test. A second dose of the vaccine was offered to all those who displayed no seroprotective titers after the first vaccination. Antibody levels were again determined 6 weeks later. Twenty-one of 28 liver transplant patients completed the study. The seroconversion rate was 62% after the first dose and 89.5% after the second dose. At baseline, 7 of 21 patients (33.4%) were already seropositive. Increasing time since transplantation positively correlated with successful seroconversion. In conclusion, a single dose of a pandemic influenza A vaccine does not elicit a reliable immune response in adolescent pediatric liver transplant patients. A second dose of the vaccine is warranted in this group of patients, at least in a pandemic scenario. There is an urgent need to further assess vaccine strategies in this high-risk group. Liver Transpl 17:914–920, 2011. © 2011 AASLD.