The influence of knowing someone with HIV/AIDS on HIV preventive behaviors has become increasingly relevant in the literature, with controversial findings. The aim of this study was to investigate the relationship between knowing someone with HIV/AIDS and preventive behaviors in a representative sample of the Italian adult population. Drawing on two sociocognitive models, perceived threat and HIV/AIDS knowledge were proposed as mediators of this relationship. Results from 1969 telephone interviews were analyzed. Questions included sociodemographic information, knowing someone with HIV/AIDS, past sexual behaviors, HIV/AIDS knowledge, HIV/AIDS perceived threat (susceptibility and severity), and preventive behaviors (HIV testing and condom use). The results of mediation models showed that knowing someone with HIV/AIDS had an indirect effect on condom use through HIV-perceived susceptibility. Knowing someone with HIV/AIDS showed a direct and an indirect effect through HIV/AIDS knowledge on HIV testing.