OBJECTIVES: In its guidance on the use of PRO measures, the FDA recommends that sponsors provide evidence that the content validity and other measurement properties are adequately similar across all versions used in a clinical trial. One way to ensure the comparability of content is to ensure that all translations reflect the original item intent. As this presupposes the existence of a formal description of each item concept, it is the objective of this study to investigate if this was the case prior to the guidance publication and to make recommendations on the basis of our findings. METHODS: The research is based on the review of PRO translations performed between 1995 and 2009. RESULTS: A total of 640 questionnaires (27 generic and 613 disease- or condition-specific) were translated. All were developed before the publication of the FDA’s PRO guidance. None had a formal written document listing a definition of each item concept and possible translation alternatives available before launching the translations. In all cases, the item-by-item concept list and the list of translation alternatives were developed in collaboration with the developers of the original questionnaires during the translation process. In some cases (multiple projects involving different languages for the same questionnaire), the development of the concept list was a dynamic process fed by questions raised at each new translation [e.g. Quality of Life in Inflammatory Bowel Disease Questionnaire (IBDQ)]. Over the years a standardized document was developed to help the authors to clarify the concepts and intent of each item of their questionnaire(s). This document and examples will be presented. CONCLUSIONS: This review demonstrates the lack of a formal, written concept lists for PRO questionnaires developed before the publication of the FDA guidance and the need to develop standardized documents. Involvement of the developer(s) is crucial at this step.