In 1990-91, 237 female sex workers from Copenhagen were enrolled in a larger study performed in 9 European countries. None of 206 women accepting serological testing was HIV-infected despite the fact that 36 (17.5%) were injecting drug users (IDU). Whereas 95% of the women always used condoms with clients over the last 6 months, this proportion was 25% and 9% respectively for casual and regular non-paying partners (P < 0.0001 for both comparisons). Those reporting at least one STD over the last year had more non-paying sexual partners than the others (P < 0.01) and the frequency of STD was lower in women who always used condoms with non-paying partners (7% vs 31%, P = 0.01). Women working on the street were more often IDU than others (78% vs 7%, P < 0.001). Independently of drug use, street prostitutes also tended to have more clients (P = 0.007) and more STD (P = 0.05). The striking differences in condom use with clients as compared to non-paying partners and the association between STD and sexual behaviours with such partners but not with clients show that specific interventions should be designed to promote safer sex with non-paying partners.