Knowledge of, and attitudes toward, AIDS were assessed in a random sample of over 2600 individuals aged 16 and over in all states and territories of Australia. Those with lower knowledge of AIDS were more likely to be separated, divorced or widowed, older, and more personally concerned about AIDS. There were no differences in knowledge of AIDS across states, or between sexes. Individuals with lower knowledge of AIDS had greater fear of homosexuals, more unrealistic concerns about AIDS, blamed those infected more, were more afraid of the unknown aspects of AIDS, and were more conservative. Individuals who had used intravenous drugs ever and in the past year had significantly lower knowledge of AIDS; for other risk behaviours, there were no significant differences. Individuals who personally knew homosexual people had higher knowledge of AIDS. These data indicate that the determinants of knowledge of AIDS are related more strongly to attitudinal variables than to demographic ones, and that there are few differences in knowledge across those practising at risk behaviours, compared with the general population, with the exception of intravenous drug users.