A method for the determination of ethyl glucuronide (EtG) in hair samples, using liquid chromatography/electrospray tandem mass spectrometry (LC/ESI-MS/MS), was developed and validated. The treatment of hair samples was as follows: to 100 mg of washed (dichloromethane followed by methanol, 1 ml each) and cut (1–2 mm) material, 700 µl of water, 20 µl of internal standard solution (pentadeuterated EtG, D5-EtG, 500 µg/l) and 20 µl of methanol were added. Samples were incubated at 25 °C overnight and then ultrasonicated for 2 h. Finally, 8 µl of the centrifuged solution (13 000 rpm) were analyzed by LC/ESI-MS/MS in negative ion mode. The surviving ions of EtG and D5-EtG were monitored together with the following MRM transitions: m/z 221 → 75, m/z 221 → 85 (EtG) and m/z 226 → 75, m/z 226 → 85 (D5-EtG). The method exhibited a mean correlation coefficient better than 0.9998 over the dynamic range (3–2000 pg/mg). The lower limit of quantification (LLOQ) and the limit of detection (LOD) were 3 and 2 pg/mg respectively. The intra- and interday precision and accuracy were studied at four different concentration levels (3, 5, 56 and 160 pg/mg) and were always better than 7% (n = 5). Matrix effects did not exceed 20%. The method was applied to several hair samples taken from autopsies of known alcoholics, from patients in withdrawal treatment, from social drinkers, from adult teetotalers and from children not exposed to ethanol, with EtG concentrations globally ranging from ≤2 to 4180 pg/mg. Copyright © 2005 John Wiley & Sons, Ltd.