The aim of this study was to develop a hypothesis to explain the link between HIV prevalence and area of residence. The study was conducted in two parts using two existing data sources. In Part 1, the bloodborne viral test status and test results of a sample of clients attending treatment in December 2001 in two areas of Dublin, an inner city area (Dublin 8) and a suburban area (Dublin 24), were extracted from the Bloodborne Viral Status Dataset created by Grogan. In Part 2 the characteristics of heroin users seeking treatment for the first time at treatment services in their respective areas of residence, Dublin 8 or Dublin 24, between 1997 and 2000 were examined, using data from the National Drug Treatment Reporting System. A higher proportion of heroin users in Dublin 8 had HIV and hepatitis C than did their counterparts in Dublin 24. The analysis suggests that heroin users in Dublin 8 were more likely both to have ever used cocaine and to have used heroin daily, than were those who lived in Dublin 24. Also, a higher proportion of injectors living in Dublin 8 used heroin and cocaine concurrently than did their counterparts in Dublin 24. In both samples, heroin users who lived in Dublin 8 were older than those who lived in Dublin 24. The findings led to a hypothesis:'The risk of acquiring HIV is associated with area of residence and may be linked to cocaine use.