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Background Respondent-driven sampling (RDS) is a method for recruiting hidden populations, such as people who inject drugs (PWID). In RDS, participants recruit their peers into the study; who recruited who into the study is tracked, and thus information is gathered on the population's social networks. The purpose of this study was to use information collected from an RDS study of PWID to determine the size and structure of injecting networks and whether network characteristics are associated with sharing injecting equipment. Methods A study was launched in Sydney, Australia in 2009 with five seeds, who were asked to recruit three participants each into the survey. This process was repeated until the target sample size was reached. The median size of injecting networks and the homophily (a measure of in-group affiliation) of different subgroups were calculated. Participants’ information was linked with that of their recruiter to form dyads, and multivariate analysis was conducted to determine whether dyad and injecting network characteristics were associated with sharing injecting equipment within the dyads. Results The injecting networks were large, the lowest median subgroup network size being 12. Homophily estimates indicated a lack of strong ties both within and across groups. In the multivariate analysis, factors significantly associated with sharing injecting equipment within dyads were feeling very close to their recruiter and having one or both members of the dyad identify as Aboriginal or Torres Strait Islander and one or both members having not been tested for hepatitis C in the previous year. Conclusion RDS provided valuable information on injecting networks in Sydney. PWID were shown to be socially connected with a large number of other injectors, and affiliations were formed without regard to demographic or drug use characteristics. Linking information from the recruits with that of their recruiter was a useful way of organizing information to gain a more complete understanding of risk behaviour.