According to a nationally representative sample of 3,321 men aged 20-39 surveyed in 1991, men appear well aware of the severity of AIDS: Nearly all know that AIDS destroys the immune system and that there is no cure for the disease, but a substantial minority do not think that AIDS will necessarily result in pain and death. Men's perceptions of the disease's severity seem to have little impact on their sexual behavior, with no clear relationship between men's knowledge of AIDS and their recent number of sex acts, their condom use or their participation in anal or casual sex. Men's perceptions of the general risk of human immunodeficiency virus (HIV) transmission also appear to have little impact either on their concerns about AIDS or on their behavior, but their perceptions about the AIDS rate in their local community do affect their concerns and behavior. Men know that certain kinds of behavior place them at risk, and their prior behavior significantly influences their perceptions of their own HIV risk. However, speculation about their own HIV status is only moderately related to their recent sexual behavior.