OBJECTIVE: To determine whether a clinical organization strategy can improve measles-mumps-rubella vaccination rates in adolescents by reducing missed opportunities. DESIGN: Nonequivalent control group design with a treatment clinic and a comparison clinic. Measurements were made for 8 months before intervention and for 9 months after intervention. SETTING: Two urban family practice clinics serving a low socioeconomic population. POPULATION: All clinic visits by adolescents aged 11 to 18 years. INTERVENTION: A clinic organizational strategy that systematically identified and vaccinated adolescents seen in the clinic for any reason. MAIN OUTCOME MEASURES: Two vaccination indexes were measured monthly at each clinic: the percentage of eligible visits at which measles-mumps-rubella vaccination was given and the percentage of visits that were up to date, with adolescents having received the measles-mumps-rubella vaccine at that visit or at a previous visit. RESULTS: The intervention resulted in a significant increase at the treatment clinic in the mean percentage of vaccinations given (from 7.8% to 17.4%) (Student's t = 3.02; P = .0087) and the mean percentage of up-to-date visits (from 17.2% to 38.6%) (t = 8.33; P < .0001). During the same period, there was no significant change in the mean vaccination rates in the comparison clinic. CONCLUSIONS: An organizational strategy appears to be a useful method to reduce missed opportunities and improve measles-mumps-rubella vaccination rates in adolescents from a low socioeconomic population.