Several different companies now sell ‘DNA ancestry’ or ‘ethnicity’ testing kits via the internet. A small sample of a person’s blood or saliva can be sent via the post, its DNA extracted, and a panel of polymorphic genetic markers can be analysed. This information is then used to provide a breakdown of a person’s ‘racial origins’ by categorizing someone as a percentage of their ancestry that is African, East Asian, Native American or European. Whilst these kits have proved very popular with adults interested in genealogy, we have recently become aware of their use in adoption and fostering cases in attempts to determine a child's ethnicity. We believe such use is inappropriate and indicates both a misunderstanding of the concept of ethnicity and the technical limitations of such genetic tests. We urge extreme caution in their use in any adoption and fostering decisions.