Increased incidence of HIV/AIDS in Latinos warrants effective social marketing messages to promote testing. The Tú No Me Conoces (You Don't Know Me) social marketing campaign promoted awareness of HIV risk and testing in Latinos living on the California-Mexico border. The 8-week campaign included Spanish-language radio, print media, a Web site, and a toll-free HIV-testing referral hotline. We documented an increase in HIV testing at partner clinics; 28% of testers who heard or saw an HIV advertisement specifically identified our campaign. Improved understanding of effective social marketing messages for HIV testing in the growing Latino border population is warranted.