In bone marrow (BM) biopsies, tartrate-resistant acid phosphatase (TRAP) staining represents the gold standard for the characterisation of osteoclasts. TRAP is one of the few enzymes that is histochemically detectable on formalin-fixed paraffin-embedded tissue. This study investigated whether TRAP is also able to visualise BM osteoclasts in autopsy tissue. It was hypothesised that, due to a progressive loss of enzymatic activity in osteoclasts post-mortem, TRAP staining could allow the time of death of a patient to be determined.TRAP-stained BM slides of 96 cases including 51 pathology and 23 forensic autopsies and 22 biopsies were histologically evaluated and their staining intensity (SI) semi-quantitatively graded. In the autopsy cases, the results were correlated with the post-mortem interval (PMI, time span in days between death and autopsy).TRAP staining intensities (TRAP-SIs) did not differ between men and women and showed a steady decrease with age. TRAP-SIs were significantly stronger in biopsies than in autopsy cases. Among the autopsies, TRAP-SIs were highly variable and not dependent on PMI, except for three forensic cases with PMI ?7 days which showed a complete loss of TRAP stainability. On the whole, the TRAP-SIs of pathology and forensic cases did not differ significantly.This study clearly shows that BM osteoclasts stay TRAP-positive for 7 days post-mortem, although with markedly reduced TRAP-SIs compared with biopsies. Since TRAP-SIs were not correlated with the duration of PMI, TRAP staining of BM osteoclasts cannot serve as a tool to determine the time of death of a patient.