To understand and improve adolescent decision making about STD prevention, there is a need to examine the initial phase in the decision-making process, namely adolescents' perceptions of the set of possible prevention measures. A relatively diverse sample of 48 sexually active adolescents answered open-ended interview questions designed to elicit STD prevention measures considered spontaneously. These adolescents produced 119 different STD prevention measures which fell into 16 different substantive categories. The average adolescent knew of over 13 prevention measures, 73% of which were individual actions and 27% of which were societal actions. Using condoms and abstaining from sexual activity were by far the most widely known prevention measures. The only measure that a majority of these adolescents (87%) have personally used is condoms. The most effective measures were not necessarily those that come to mind most readily. Several significant gender differences in the salience of prevention measures were found.