OBJECTIVES: To identify unique, mutually exclusive population segments whose members share characteristics associated with likelihood of influenza vaccination. METHODS: Data from 30,668 elderly participants in the 1999 Behavioral Risk Factor Surveillance Systems was analyzed using classification and regression tree analysis. RESULTS: Eleven subgroups were identified: Non-Hispanic Blacks and Hispanics with no recent checkup had the lowest prevalence estimate (28.1%), whereas married white persons aged 75 plus with a recent checkup had the highest (80.6%). CONCLUSIONS: Influenza vaccination varies greatly according to preventive medical care use and race/ethnicity, factors that are amenable to targeted outreach efforts.