The role of bias in assessments of personal susceptibility to threat is a central concept in research on perception of risk. The current study aimed to clarify the association between perceived personal susceptibility to infection with HIV/AIDS and injecting risk behaviour with injecting drug-users' perception of the baseline rate of infection with HIV/AIDS. 1262 injecting drug-users from Australian cities were interviewed. The injecting drug-users were divided into high- and low-risk groups depending on the HIV/AIDS risk associated with their injecting behaviour. Subjects were subdivided into low-, medium-, and high-perceived personal susceptibility groups. Analysis indicated that injecting drug-users in the high-risk group underestimated the prevalence of HIV/AIDS infection relative to those in the low-risk group and that perceived personal susceptibility was rationally related to estimates of the baseline rate of infection.