Perceptions of their own and others' risk of contracting AIDS, AIDS knowledge, and extent of AIDS-preventive behavior (APB) were assessed in sexually active heterosexual college students (N = 80) to investigate why many college students fail to enact APB. Personal risk of contracting AIDS was estimated to be significantly lower than risk for each of a set hypothetical persons who varied in degree of similarity to respondents, p < .0001. ANOVAs indicated that males (F = 60.13, p < .0001), homosexual males and females (F = 62.55, p < .0001), and young adults not in college (F = 8.17, p < .005) were perceived as more likely to contract AIDS than females, heterosexuals, and college students, respectively. AIDS knowledge was related to accuracy of risk estimates, but was not predictive of APB. Lack of APB is discussed in terms of illusion of unique invulnerability and the characterization of AIDS as an outgroup problem. Implications and recommendations for AIDS-preventive education programs are discussed.