OBJECTIVE: To examine the nature and extent of ambulance attendances involving gamma-hydroxybutyrate (GHB) and to compare these with heroin-related attendances in Melbourne, Victoria. DESIGN: Retrospective analysis of a database of ambulance service records on attendances at non-fatal drug overdoses, March 2001-October 2005. PARTICIPANTS AND SETTING: Patients who took GHB and were attended to by an ambulance, as recorded by Metropolitan Ambulance Service (Melbourne) paramedics. MAIN OUTCOME MEASURES: Transportation to hospital by ambulance; other outcomes included number, age, sex and Glasgow Coma Score (GCS) of patients, characteristics of attendances (in public or private space, others present, police co-attendance). RESULTS: There were 618 GHB-related ambulance attendances across the 46 months of data collection; 362 involving GHB only and 256 involving the concurrent use of GHB and other drugs. These figures compare to 3723 heroin overdoses observed during the same period. The number of GHB-related attendances increased by around 4% per month, which was a higher rate of increase than that found for heroin overdose attendances. Most patients were younger than 25 years, were attended in public spaces, and had a GCS <10. Around 90% of patients were transported to hospital, compared with 21% of heroin overdose attendances. CONCLUSIONS: Ambulance attendance data can be used to index GHB-associated harms. The clear increases in GHB-related ambulance attendances over time highlights the need for further research on how best to respond to this emergent drug-related harm.