Utilizing the concept of race-based traumatic stress, this study tested whether posttraumatic stress symptoms explain the process by which perceived discrimination is related to health risk behaviors among Mexican American adolescents. One hundred ten participants were recruited from a large health maintenance organization in Northern California. Mediational analyses indicated that adolescents who perceived more discrimination reported worse posttraumatic stress symptoms, controlling for covariates. In turn, adolescents who experienced heightened posttraumatic stress symptoms reported more alcohol use, more other drug use, involvement in more fights, and more sexual partners. Perceived discrimination was also directly related to involvement in more fights. Results provide support for the notion of race-based traumatic stress, specifically, that perceived discrimination may be traumatizing for Mexican American adolescents. Counseling psychologists and counselors in schools and community settings should assess Mexican American adolescents for the effects of discrimination and provide appropriate interventions to reduce its negative emotional impact. (Contains 1 figure and 3 tables.)