Background: Estimates suggest that about 48% of nearly 19 million cases of sexually transmitted diseases (STDs) occurring annually in the United States are acquired by persons aged 15-24 years. The purpose of this study was to test the hypothesis that adolescents' attitudes about protecting themselves from STDs predict their laboratory-confirmed prevalence of STDs in early adulthood. Methods: Wave 3 of the National Longitudinal Study of Adolescent Health assessed "Chlamydia trachomatis", "Neisseria gonorrhoeae", and "Trichomonas vaginalis." This wave 3 data were regressed on data collected in wave 1 (when those followed were teens). A single-item measure (with a 5-point response option) assessed adolescents' attitude: "It would be a big hassle to do the things necessary to completely protect yourself from getting an STD." Results: Valid urine specimens were provided by 8297 adolescents who also completed the self-reported measures needed for this study. Overall, 6.4% of the weighted sample tested positive for at least 1 of the 3 STDs. Controlling for age, gender, minority status, and age of sexual debut (all of which are well-established predictors of STD prevalence), attitude toward STD protection achieved significance (P less than 0.001). Each additional point on the 5-point scale increased adolescents' odds of testing positive for an STD in early adulthood by about 13%. Of interest, the attitudinal measure did not interact with any of the other variables. Conclusions: Findings provide evidence suggesting that safer sex programs may benefit adolescents by fostering positive attitudes toward practices that avert STD acquisition. (Contains 2 tables and 1 figure.)