Research studying HIV risk-taking behaviour has tended to focus on specific populations who are characterized by particular behaviours. Such practices include the extent of unprotected anal intercourse among homosexual men, the sharing of injecting equipment among drug users and unprotected vaginal intercourse among female sex workers. There is often a failure among both researchers and practitioners to address specific risk behaviours outside of the defined risk group. Reviewed here are studies of heterosexual anal intercourse, with special attention to drug users and female sex workers. Among the general population, the reporting of heterosexual anal intercourse varies greatly across time and cultural groups. However, a body of recent research suggests that, despite some geographical differences, rates in most countries are relatively high. Many studies have also found that associated condom usage is low. Although unprotected anal intercourse is known to be an efficient method for the transmission of HIV and other viral infections, it has rarely been addressed outside cohorts of men who have sex with men. Of particular concern is the interaction of risk behaviours among drug users and women engaging in prostitution, many studies having found high levels of unprotected anal intercourse among these groups. Harm reduction interventions must address all the health risk behaviours their target groups are engaging in, if the challenge of reducing the transmission of HIV and other viral infections is to be met.