Quetiapine, which belongs to a new chemical class, the dibenzothiazepines, is structurally related to clozapine. It has a high affinity for 5-HT2 receptors and a low affinity for D1 and D2 dopamine receptors. Because of its recent release to the U.S. market, there is little information on its therapeutic, toxic, and lethal concentrations. In the 13 postmortem cases examined in the current study, quetiapine was identified and quantitated by capillary gas chromatography with nitrogen phosphorus detection following a basic liquid-liquid extraction. Confirmation was achieved by full scan electron impact gas chromatography/mass spectrometry. Heart blood quetiapine concentrations ranged from 0.07 to 18.37 mg/l and femoral blood concentrations ranged from 0.06 to 19.25 mg/l. the average heart blood/femoral blood ratio was 1.31 (range 0.55 to 2.57, n=10). Urine, bile, and gastric contents were assayed in all cases in which they were submitted. In three cases the cause of death was determined to be quetiapine toxicity. In these cases, heart blood concentrations ranged from 0.72 to 18.37 mg/l (n=3). In cases where quetiapine was not considered a factor in the cause of death, heart blood concentrations ranged from 0.07 to 0.51 mg/l, which is within reported therapeutic concentrations. The authors advise that it is important that quetiapine concentrations be evaluated in the context of the entire case when determining the relative contribution of this drug to a person's death. 4 tables, 4 figures, and 13 references