The purpose of this study was to determine whether efficacy expectation and outcome expectation are significant predictors of sexually transmitted disease (STD) preventing behaviors. The study was based on Self-Efficacy Theory. Three hundred nine female college students attending a student health gynecology clinic completed a 56-item self-administered questionnaire comprised of items both used in previous research and developed by the investigator. The major variables of the study were knowledge of STD prevention, perceived risk of acquiring a STD, outcome expectation of practicing STD preventive behavior, efficacy expectation (perceived ability) to perform safe behaviors, sources of efficacy information, and sexual behavior over the six months prior to the survey. Sexual behavior indicators included condom use, number of and familiarity of partners, and partner screening. Women indicated that efficacy expectancy was lowest for refusing sexual intercourse in a variety of situations, and highest for communicating about preventing STDs. The most common sources of information on STD prevention that college women cited were friends, magazines, and television. Sources of persuasive efficacy information about STD prevention included friends, boyfriends and parents. Hierarchical regression was performed to identify whether efficacy and outcome expectation explained a significant amount of variance in sexual behavior beyond that explained by knowledge and perceived risk. Knowledge and perceived risk were entered at step one, and efficacy and outcome expectation were entered via forward procedure at step two. Knowledge and perceived risk explained a significant amount of variance in sexual behavior (F = 13.13; df = 2, 302; p < .0001), and efficacy expectancy 24% of the variance (F = 31.13; df = 3, 301; p < .0001). Outcome expectancy did not significantly explain additional variance in sexual behavior. These findings indicate that nursing interventions should be directed toward enhancing women's efficacy expectancy to prevent STDs. Methods of improving efficacy expectancy in preventing STDs include engaging in role-play to practice behaviors, observing others perform this behavior, such as through videos, and persuading women through encouragement of their ability to perform these behaviors.