To assess the prevalence and effects of alcohol and drug use in heterosexual and homosexual commercial contacts, and the relationship between their use and unsafe sexual behaviour, 127 female prostitutes, 27 male prostitutes, 91 clients of female prostitutes and 24 clients of male prostitutes were interviewed face-to-face with the help of a semi-structured questionnaire. The respondents were living or working in different parts of The Netherlands. Alcohol and drug use was found to be relatively common among prostitutes. This was also so for the use of alcohol by clients, though to a lesser extent. Prostitutes' consumption varied widely according to the type of prostitution they were employed in. Those meeting their clients in clubs or bars reported the highest consumption of alcohol; hard drugs were used predominantly by street prostitutes. It appears that the main effects of alcohol and drug use are on how the individual experiences working as, or calling on, a prostitute, the social interaction between the two parties, and the sexual contact itself. The common assumption that drinking alcohol has negative effects on condom use was not borne out; though female prostitutes working under the influence of drugs were significantly more likely to report unsafe sex. The degree to which commercial partners were judged to be under the influence of alcohol or drugs was not found to bear upon the frequency of respondents' condom use. For those prostitutes who use hard drugs, this use plays an important role in their engaging in unsafe sexual activities. Prevention activities should focus especially on this group, and should take into account the role of such drug use.