The benzodiazepine clonazepam is a prescription drug used to treat epilepsy and anxiety. In addition, it is frequently used to treat drug addicts and is itself a popular drug of abuse. In this study, we report the incidence and blood concentrations of clonazepam and its metabolite 7-aminoclonazepam in cases referred to the Section of Forensic Chemistry at the University of Copenhagen in 2002-2007. Using LC-MS/MS, clonazepam was detected in 297 traffic cases, 92 criminal cases (perpetrators or victims of a crime) and in 140 postmortem cases. The concentration ranges of clonazepam+7-aminoclonazepam were 0.002-0.840 mg/kg (median 0.067) for traffic cases, 0.005-0.913 (median 0.071) for criminal cases (offenders), 0.002-0.720 (median 0.030) for criminal cases (victims) and 0.002-1.676 (median 0.115) for postmortem cases. The concentrations were thus similar among the groups, although the median value was highest in the postmortem group. In most cases, other drugs were also present. For the postmortem group, the cases (n=27) with relatively high (>0.2 mg/kg) clonazepam+7-aminoclonazepam values were examined in greater detail. Other drugs were present in all cases, with clonazepam judged to be the primary cause of death in five cases. The range of clonazepam+7-aminoclonazepam concentrations in these five cases ranged from 0.26 to 0.54 mg/kg (median 0.29 mg/kg).