There is a need for accurate and consistent oral assessment to measure mucosal changes and oral complications associated with cancer therapies. Mucositis is an important and common side effect of cancer therapies that merits the identification of improved health-care interventions. Developing appropriate and reliable oral assessment instruments for use with children is relevant to the evaluation of these interventions. The purpose of this study was to determine the content validity of the oral assessment guide (OAG) in children: an instrument that was designed to objectively assess the physiological changes of the oral cavity following administration of chemotherapy and radiotherapy to adults. This process is considered to be most effective when undertaken systematically. A judgement quantification process was used with health care professionals in paediatric oncology to establish content validity of items (n=9) and instrument (n=10). A revised OAG more pertinent to children and young people was produced in the light of this process.