BACKGROUND : Hair samples from children are frequently analyzed in order to characterize their endangerment in a drug using environment. However, the interpretation of the results remains difficult because of lacking data for comparison. In this study, hair samples from families with drug consuming parents were analyzed for illegal and selected medical drugs and the results were evaluated concerning a relationship between findings of parents and children depending on kind of drug, age and gender of children as well as maternal or paternal drug concentrations in hair. METHODS : In an ongoing social supporting project for families with underage children and drug consuming parents, hair samples were analyzed since 2011 for methadone, opiates and opioid analgesics, cocaine, amphetamines, ecstasy, cannabinoids and benzodiazepines by LC-MS/MS with LOQs ≈ 0.01 ng/mg. From the data pool of more than 1300 individuals, 100 families with results for one or both parents and one to five children, 30 families with results only for both parents, and 11 families with results only for 2-4 children were selected. Fifty eight of these 141 families were repeatedly tested (altogether 251 family tests). RESULTS : One to 5 drugs were detected in 239 (95.2%) of the family tests with highest occurrence of cocaine (79.7%) and THC (50.2%). According to the concentrations of the tested persons, the most probable drug users were the mother (25%), the father (24%), both parents (16%), or were not tested (30%). Within the families, there was an agreement of the detected drugs between parents and children of 47.8%, between both parents of 36.1%, and between children of 42.3%. For parents with hair concentrations in the typical range of regular drug use, the drug was detected in children hair with the following frequency: methadone 65.5%, heroin (6-AM) 63.6%, cocaine 92.1%, amphetamine 80%, MDMA 42.9% and THC 67.4% with higher percentage for younger children. The agreement for medical drugs (benzodiazepines 7.7%, synthetic opioids 8.7%, diphenhydramine 7.1%) was much lower suggesting voluntary administration or intake. Despite the strong variation of the data, clear trends were found that the child/parent drug concentration ratio decreases with increasing children age and is higher for boys than for girls. CONCLUSIONS : The comparison of hair results within families gives a deeper insight in the drug situation, often enables the identification of the drug user and is helpful for social and legal decisions to improve the conditions of the children.