Recent studies suggest that many fatal heroin overdoses are caused by anaphylactoid reaction. In the present study we measured tryptase and eosinophil cationic protein in post-mortem blood of 48 deaths after heroin injection. We also investigated the presence and pulmonary distribution of mast-cells using specific immunohistochemical antibody for tryptase and morphometric evaluation in those cases of heroin-related deaths. The data were compared with 44 subjects who died following head trauma and to 32 cases of fatal anaphylactic shock. In the heroin-related death cases, the measurements of serum tryptase levels and eosinophil cationic protein dosages resulted in particularly elevated concentrations compared with the trauma cases. Nevertheless, the data that our study supplies by immunohistochemical techniques indicate that when mast-cells count in the lung was determined, no definite pattern was obtained between fatal heroin overdose cases and the control groups. Furthermore, the wide range of morphine concentrations found in post-mortem blood samples suggest that the term 'overdose' is relative and does not sufficiently characterize death associated with heroin addiction. Our study confirms that elevated concentrations of serum tryptase are associated with many heroin-related deaths. At this moment to attribute the cause of these deaths to 'heroin overdose' ignores the likely causal contribution of other possible systemic reactions to the mechanism of death.