Collagenous colitis is a recognised cause of persistent, non-bloody, watery diarrhoea. There are few cases of collagenous colitis reported in children or occurring within families. To our knowledge, no familial cases under 14 years of age have been reported previously; we describe a case of familial collagenous colitis affecting a 6-year old girl and her mother. The relevant published literature is reviewed and management is discussed. Colonic mucosal biopsies should be considered in both adults and children presenting with persistent watery diarrhoea even in the absence of any macroscopic abnormality at colonoscopy.