Theories of preventive health behavior posit that perceived vulnerability to health threats motivates self-protective behavior. Fifteen years after an initial study of college students' perceptions of their vulnerability to HIV, a replication was conducted on the same campus in 2005. Comparisons between cohorts on vulnerability judgements were conducted to examine whether, and to what extent, student perceptions had changed across time. Respondents in the 1990 study had judged themselves as less vulnerable to HIV/AIDS than others about whom they also made risk estimates; this pattern was replicated among 2005 respondents, even though many in both cohorts were involved in objectively risky practices. Comparisons between the cohorts on risk behaviors revealed that fewer of the 2005 students reported being sexually active, and those who were active used condoms more frequently. However, 2005 respondents also reported more recreational drug use, and oral and anal sexual activity than the 1990 students.