This study investigates the perceived risk of an HIV infection. Cognitive antecedents of biases in risk perception and their effects on behavioral intentions were investigated. The purpose of this study was to determine the relative accuracy of risk assessments for samples that vary in actual risk for AIDS. Subjects were citizens of Amsterdam, heterosexuals with private partners, gay men, and heterosexuals with prostitution partners. Although optimistically biased in all samples, perceptions of risk were related to previous risk behavior in high-risk samples only. Pessimism was more pronounced in samples higher at risk. Optimists had lower levels of previous risk behavior and increased intentions to adopt safe sex practices. Ss in samples higher at risk had, therefore, relatively adequate perceptions of risk.