An outbreak of unexplained illness among heroin users was reported in May 2000 in Dublin. Between 29 April and 26 June 2000, 22 cases were identified, with eight deaths. This occurred in association with similar outbreaks in the UK, particularly Glasgow. Illness was characterised by soft tissue inflammation at the injection site, followed by hypotension and circulatory collapse. Laboratory results indicated that clostridium species were associated with the outbreaks. Analysis of cases in the UK indicate increased risk for female injectors and longer term users. However, data on injecting practice were unavailable. Injecting intramuscularly has been associated with outbreaks of wound botulism and tetanus. A prospective case-control study was initiated to identify risk factors associated with this outbreak.