OBJECTIVE The goals were (1) to describe immunization rates for tetanus-diphtheria, hepatitis B, and measles-mumps-rubella vaccines among 13-year-old adolescents; (2) to identify missed opportunities for tetanus-diphtheria immunization among adolescents 11 to 17 years of age; and (3) to evaluate the association between preventive care use and tetanus-diphtheria immunization. METHODS Adolescents born between January 1, 1986, and December 31, 1991, and enrolled in Harvard Pilgrim Health Care and Harvard Vanguard Medical Associates for >or=1 year in 1997-2004 were included. Immunization rates for tetanus-diphtheria, hepatitis B, and measles-mumps-rubella were assessed at 13 years of age. Missed opportunities for tetanus-diphtheria immunization within 14 days after a health care visit were measured. Multivariate models were used to determine predictors of timeliness of tetanus-diphtheria vaccination, particularly the use of preventive care services. RESULTS. A total of 23,987 eligible adolescents were enrolled in Harvard Pilgrim Health Care and Harvard Vanguard Medical Associates between 1997 and 2004. Among 13-year-old adolescents in the most recent birth cohort, 84%, 74%, and 67% were up to date for tetanus-diphtheria, hepatitis B, and measles-mumps-rubella, respectively. When the analysis was limited to those with >or=1 vaccine received before 2 years of age (a proxy measure for complete records), 92%, 82%, and 85% were up to date for tetanus-diphtheria, hepatitis B, and measles-mumps-rubella, respectively. Missed opportunities for tetanus-diphtheria immunization occurred at 84% of all health care visits. Adolescents who did not seek preventive care were less likely to receive tetanus-diphtheria in a timely manner. CONCLUSIONS Adolescent immunization rates lag far behind childhood rates, and missed opportunities are common. Additional strategies are needed to increase the use of preventive services among adolescents and to enable providers to vaccinate adolescents at every opportunity.