Injection drug users who exchange sex for money or drugs may serve as a bridge group for transmitting HIV between injectors and non-injectors. While many individual characteristics have been linked to exchanging sex, little attention has been given to the influence of social network members. The present study assessed the relationship between exchanging sex and perceptions of peers' sex exchange behaviour and attitude toward sex exchange. The sample was composed of 267 women heroin and cocaine injectors in Baltimore, MD, USA. The results indicate that women who believed that their friends exchanged sex were more twice as likely to exchange sex in the past 90 days (95% CI: 1.49-2.70). Also, participants who thought their peers disapproved of sex exchange were 20% less likely to exchange sex (95% CI: 0.67-0.95). These findings suggest the need for peer education interventions that promote norms about safer behaviours.