Although risk perception as a motivator of precautionary behavior is a key component of several health behavior theories, this motivational hypothesis has found mixed support in the HIV/AIDS area. This may be, in part, because risk perceptions are more complex than they are treated in many studies of the motivational hypothesis. The current study examines demographic, personality, and sexual risk factors as predictors of partner-specific (main vs. casual) HIV/STD risk perceptions in a sample of 1,489 young adults. As expected, perceptions of HIV/STD risk were higher in the context of "casual" as compared with "main" partnerships. Although univariate analyses demonstrated that gender, race/ethnicity, sensation seeking, impulsivity, number of partners, and condom use all influenced HIV/STD risk perceptions, only gender, condom use, and race/ethnicity remained significant in multivariate analyses. Implications of these results for the design of efficacious HIV prevention interventions are discussed.