<b><i>Background/Aims:</i></b> Social networks have been hypothesized to protect people from the harmful effects of stress, but may also provide dysfunctional role models and provide cues associated with drug use. This study describes the range, type and level of social support available to patients engaged in UK opiate substitution treatment (OST) programmes, and explores the association between network factors and continued use of illicit heroin. <b><i>Methods:</i></b> A cross-sectional survey of a randomly selected sample of OST patients (n = 118) utilised measures of current substance use and social network structure and support. <b><i>Results:</i></b> More than half of the participants had used heroin in the previous month, and most described networks that were both supportive and positive about treatment. Multivariate analysis showed that the substance use involvement of network members was higher in those patients still using heroin, even when other treatment factors were controlled for. <b><i>Conclusion:</i></b> There was a strong association between ongoing contact with other drug users and continued use of illicit heroin in this treatment sample. Whilst there is potential for the involvement of social networks in treatment, future research needs to ascertain the exact nature of the relationship between social support and drug use.