Research findings in psychoneuroimmunology document reliable, bidirectional linkages among psychological processes, the nervous system, and the immune system. However, available data are based almost entirely on animal and adult human studies; the application to children and adolescents is uncertain. We capitalized on the experimental leverage provided by a routine vaccination to examine the link between mood symptoms and the immune response to a vaccine challenge in early adolescence. One hundred twenty-six 11-year-olds for whom vaccine response data were available were assessed at prevaccination and 4 weeks, 3 months, and 6 months following vaccination; self-report ratings of depression and anxiety as well as measures of psychosocial and somatic risk were assessed prior to vaccine response. Analyses indicated that children's internalizing mood symptoms were associated with elevated and persistently higher antibody responses, with evidence extending to two of the four serogroups. The associations remained after controlling for multiple possible confounders (social class, body mass index, sleep, psychosocial risk, and pubertal status). The observed enhanced vaccine response associated with depressive and anxious symptoms in early adolescence may reflect an important developmental difference in immune system-brain interplay between adults and children, and it underscores the need for further developmental studies of psychoneuroimmunology.