In some subjects with severe neurological diseases, a reduced immune response to seasonal influenza vaccine has been demonstrated. Patients with Williams or Cornelia de Lange syndrome frequently have abnormalities in neurodevelopment. This study has evaluated the immunogenicity, safety and tolerability of a monovalent 2009 pandemic influenza A/H1N1 MF59-adjuvanted vaccine in these subjects. Eighteen patients with Williams syndrome (ten males; mean age ± standard deviation [SD] 12.74 ± 4.49 years), 11 with Cornelia de Lange syndrome (six males; mean age 12.90 ± 4.85 years) and 30 age- and gender-matched healthy controls (16 males; mean age 12.49 ± 4.55 years), never vaccinated against influenza, received a dose of the vaccine between 1 and 30 November 2009. Four weeks later, the seroconversion rates in the three groups were between 72% and 80% and the seroprotection rates were 100%, with a similar increase in antibody levels. Two months later, most of the subjects remained seroconverted with no statistically significant difference between the groups, and about 94% of the patients with Williams syndrome, all of those with Cornelia de Lange syndrome and all of the healthy controls were still seroprotected. Safety and tolerability were very good, with no difference between the groups. None of the patients developed documented influenza during the study period. These results show that the immunogenicity, safety, and tolerability of a single dose of the monovalent 2009 pandemic influenza A/H1N1 MF59-adjuvanted vaccine in children and adolescents with Williams or Cornelia de Lange syndrome and moderate to severe mental disabilities is very good, and similar to that of healthy subjects.