Abstract: The demographic and toxicological characteristics of deliberate (SUI, n = 50) and accidental (ACC, n = 927) fatal heroin overdose cases were examined. SUI cases were more likely to be female, had lower body mass indices, were more likely to be enrolled in treatment and less likely to have hepatic pathology. The median blood morphine concentration of SUI cases was significantly higher than that of ACC cases (0.70 vs. 0.40 mg/L, p < 0.001). Blood morphine concentrations of >1 mg/L were seen among 38.0% of SUI cases compared to 13.9% of ACC cases. Being a member of the SUI group remained a significant independent predictor of higher morphine concentrations after controlling for the effects of potential confounders (p < 0.001), other significant predictors being the absence of alcohol (p < 0.001), the presence of methadone (p < 0.05), and the presence of cocaine (p < 0.05). The current data are consistent with the view that suicide forms a small, but distinct, category of heroin overdose cases, rather than overdose being a parasuicidal phenomenon per se.