Clobazam (Castillium, Urbanil), a benzodiazepine often used as an anxiolytic and in the treatment of epilepsy, is considered a relatively safe drug. The authors present a fatal case with a 49-year-old female, found dead at home. She had been undergoing psychiatric treatment and was a chronic alcoholic. The autopsy findings were unremarkable, except for multivisceral congestion, steatosis and a small piece of a plastic blister pack in the stomach. Bronchopneumonia, bronchitis and bronchiolitis were also diagnosed. Anhigh-performance liquid chromatography (HPLC)/diode array detector (DAD)/mass spectrometry detection (MSD) with electrospray method was developed in order to detect, confirm and quantify clobazam in the post-mortem samples. In the chromatographic separation, a reversed-phase column C18 (2.1 x 150 mm, 3.5 microm) was used with a mobile phase of methanol and water, at a 0.25 ml/min flow rate. Carbonate buffer (pH 10.5) and 20 microl of prazepam (100 microg/ml) as internal standard were added to the samples. A simple and reliable liquid-liquid extraction method for the determination of clobazam in post-mortem samples was described. Calibration curves for clobazam were performed in blood, achieving linearity between 0.01 and 10 microg/ml and a detection limit of 1.0 ng/ml. The clobazam concentration found in post-mortem blood was 3.9 microg/ml, higher than the reported therapeutic concentration (0.1-0.4 microg/ml). The simultaneous acquisition by photodiode array detection and mass spectrometry detection results allowed benzodiazepines to be identified with sufficient certainty. An examination of all the available information suggested that death resulted from respiratory depression due to clobazam toxicity.