Alcohol is the most consumed substance of abuse in our environment and at a premature age. Alcohol consumption and abuse in adolescents can lead to a number of health and social issues. Determination of ethanol consumption in adolescents by questionnaire is not a reliable method of screening. Biomarkers of ethanol in alternative biological matrices are useful tools in prevalence studies.The aim of our study was to establish the real prevalence of alcohol consumption in preadolescents and adolescents through the evaluation of ethyl glucuronide, an ethanol biomarker, (EtG) in nails. We conducted a prospective study in 127 children between 11 and 16 years at Hospital del Mar (Barcelona) and 107 adolescents between 13 and 18 years at Hospital Virgen de la Arrixaca (Murcia) who consulted for any reason at the Emergency Department. They answered a questionnaire about alcohol and other drugs of abuse consumption and then samples of fingernails were obtained. Twenty-nine samples in Barcelona (22.8%) and 21 samples in Murcia (20%) were positive for EtG. A total of 93.1% of patients that were positive for EtG in nails in Barcelona and 33.3% in Murcia didn’t report alcohol consumption in the questionnaire. In conclusion, we found that alcohol consumption in preadolescents and adolescents in our area appeared to be a topic of concern and questionnaires fail to detect chronic consumption of alcohol in the majority of cases. For this reason we suggest EtG determination in nails could be a screening tool for chronic and sustained alcohol use in adolescents.