Functional constipation and functional non-retentive faecal incontinence are common problems in childhood; these functional defaecation disorders are diagnosed according to the Rome IV criteria. Here we describe the role of an abdominal X-ray in diagnosing functional defaecation disorders, and address the limited additional value of a plain abdominal X-ray. In limited cases it may be helpful to determine colonic transit time using an abdominal X-ray to differentiate between functional constipation and functional non-retentive faecal incontinence. The diagnostic value of abdominal X-rays in children with functional abdominal pain will be discussed. The aims of this article are to explore ways of reducing unnecessary use of abdominal X-rays and reducing unnecessary radiation exposure in children with functional defaecation disorders and functional abdominal pain.