OBJECTIVES: The lack of a common terminology for naming concepts at the family and sub-family levels has increased the reluctance of various stakeholders to accept the use of PROs for decision-making, such as in an FDA label claim.  This research examines the conceptual content of selected PRO instruments, identifies similarities and differences across instruments, and explores the extent to which the availability of a classification system or taxonomy of concepts might bring standardization to the field and thus improve communication. METHODS: The structure and conceptual content of items in existing instruments is evaluated using a grammar-like decomposition of each item along with the item- or domain-level concept assigned by the instrument developer.  Item-level concepts are also assigned a classification code using the WHO International Classification of Functioning, Disability and Health (ICF).   This code identifies a concept by its major component, a high-level concept of function, disability, or environmental factors, and by its lower-levels, as appropriate. RESULTS: Approximately 650 items from 25 disease-specific and generic instruments were evaluated according to the grammar and the ICF.  Grouping the items by ICF codes indicated a wider range of concept names assigned by the developers than by the classification system.  For example, the 21 items classifiable in the Ingestion Functions of the ICF Functions Related to the Digestive System Category were assigned either to this group level or to three sub-level categories.  The instrument developers assigned these same 21 items to 7 different concepts.    This same pattern of greater diversity of concepts assigned by developers than by the ICF was observed for other WHO categories. CONCLUSIONS: Instrument developers assigned diverse concept names to represent items that assess similar areas of functioning..  The analysis suggests directions for future development of a taxonomy appropriate for classifying health-related concepts used for making statements about treatment benefit.