A highly specific and selective analytical method using LC/MS/MS for the quantitative determination of ethyl glucuronide (EtG) in urine was developed and fully validated. Since the determination of EtG in urine may be possible days after the elimination of alcohol, it is an indication of alcohol use in alcohol treatment programs and antemortem and postmortem toxicological investigations. Propyl glucuronide (PrG), which increased the selectivity of the method, was used as an internal standard. The method was validated in terms of selectivity, linearity, LOD, LOQ, intraday and interday precision, and recovery. The analyte and PrG in the SPE cleaned up extract were separated on a 150 mm C-18 column in 3.3 min with high resolution. The rest of the peaks from the matrix were eluted in 9.0 min. The LOD was 90.8 ng/mL and the LOQ was calculated using the EURACHEM method as 185.0 ng/mL. The intraday and the intermediate precision of the method was calculated using analysis of variance and confirmed with calculation of HorRat values, which were found within acceptable limits. The method provided a reliable solution for monitoring patients under alcohol addiction treatment and was successfully applied to real samples.