Hair testing was used to investigate the prevalence of unsuspected exposure to drugs of abuse in a group of children presenting to an urban paediatric emergency department without suggestive signs or symptoms. Hair samples were obtained from 114 children between 24 months and 10 years of age attending the emergency room of Hospital del Mar in Barcelona, Spain. Hair samples from the accompanying parent were also collected. The samples were analyzed for the presence of opiates, cocaine, amphetamines, and cannabinoids by ultra-performance liquid chromatography-tandem mass spectrometry. Parental sociodemographics and possible drug of abuse history were recorded. Hair samples from twenty-three children (20.1%) were positive for cocaine (concentration range 0.15-3.81 ng/mg hair), those of thirteen children (11.4%) to cannabinoids (D9-THC concentration range 0.05-0.54 ng/mg hair), with four samples positive to codeine (0.1-0.25 ng/mg hair), one positive for 2.09 ng methadone per mg hair and one to 6-MAM (0.42 ng/mg hair) and morphine (0. 15 ng/mg hair) . In 69.5 and 69.2% of the positive cocaine and cannabinoids cases respectively, drugs was also found in the hair of accompanying parent. Parental sociodemographics were not associated with children exposure to drugs of abuse. However, the behavioural patterns with potential harmful effects for the child's health (e.g., tobacco smoking, cannabis, benzodiazepines and/or antidepressants use) were significantly higher in the parents of exposed children. In the light of the obtained results (28% overall children exposure to drugs of abuse) and in agreement with 2009 unsuspected 23% cocaine exposure in pre-school children from the same hospital, we support general hair screening to disclose exposure to drugs of abuse in children from risky environments to provide the basis for specific social and health interventions.