Two methods for the determination of ethylglucuronide (EtG) in urine and in hair have been developed by liquid chromatography-tandem mass spectrometry. These two methods were fully validated, including linearity (0.25–100 µg/mL in urine; 0.05–5 ng/mg in hair; r2 > 0.99, n = 5), limits of detection (0.1 µg/mL in urine, 0.025 ng/mg in hair) and quantitation (lowest level of the calibration curve), extraction efficiency (> 55%), within-day and between-day imprecision and bias (CV and mean relative error < 15%), matrix effect, and relative ion intensity. These methods have been applied to 541 urine samples and 17 hair specimens collected from 156 alcohol withdrawal patients. The determination of ethanol versus EtG in urine was compared, and also the convenience of EtG determination in hair. EtG in urine and in hair proved to be a powerful tool for monitoring abstinence in these patients.