Dentists’ self-reported attitudes towards patients with HIV/AIDS might not reflect their actual behaviour. In this study 2 observers posed as HIV-positive patients and directly evaluated the behaviour of 300 dentists in Tehran, Islamic Republic of Iran. Two months later another observer interviewed the same dentists at their offices regarding AIDS-related knowledge and self-reported attitudes. Only 14.9% of dentists agreed to treat the simulated HIV-positive patients, 78.5% referred and 6.6% rejected them. Older age, longer work experience, graduation from a non-Iranian university and not having additional degrees were significantly related to adverse behaviours. Mean scores were 8.3 (SD 9.7) (out of 18) for knowledge and 17.5 (SD 7.1) (out of 39) for attitude. There were no significant correlations between dentists’ knowledge and attitude or between knowledge and behaviour, but there was a weak positive correlation between self-reported attitude and observed behaviour