Blood toxicology results for deaths attributed to heroin overdose during 1995 in the South Western Sydney (SWS) region (n = 39) were compared with those of a sample of 100 current SWS heroin users who had injected within the preceding 24 h. Heroin-related deaths had a higher median concentration of morphine than current heroin users (0.35 versus 0.09 mg/l). However, there was substantial overlap between the blood morphine concentrations of the two groups, ranging from 0.08-1.45 mg/l. This range incorporated 90% of heroin-related deaths. A third of current users had morphine concentrations over twice the toxic blood morphine concentration employed by the analytical laboratories, and 7% had morphine levels higher than the median recorded for fatal cases. Alcohol was detected in 51% of fatal cases (median = 0.10 g/100 ml) compared with 1% of current heroin user. There was a significant negative correlation among fatal cases between blood morphine and blood alcohol concentrations (r2 = -0.41). There was no significant difference between groups in the proportions of subjects positive for blood benzodiazepines. The results raise questions about the mechanisms of death in what are termed overdoses, and about the role of alcohol in these fatalities.