This study examines the validity of intuitive classifications of experienced diagnosticians, that is, the empirical foundation of their intuitive prototypes. Intuitive prototypes of 10 experienced diagnosticians were collected in two tasks and were compared with the empirically based assessment system of T. M. Achenbach (T. M. Achenbach, K. C. Conners, H. C. Quay, F. C. Verhulst, & C. T. Howell, 1989). Results show that the intuitive prototypes correspond very poorly with the empirical core syndromes, instead consisting of different groups of symptoms from various core syndromes. The study also showed a low similarity between behaviors that clinicians judge as co-occurring with an incompletely specified syndrome and behaviors that empirically co-occur with that syndrome. Reasons for the difference between intuitive prototypes and empirically derived classification structures are discussed, as are consequences for diagnostic practice.Research on intuitive prototypes in psychopathology has demonstrated that the prototype view corresponds to how clinicians actually think about and use diagnostic categories (Cantor, Smith, French, & Mezzich, 1980). Prototypes can be regarded as classes or categories in which features are organized. In the prototype view of classification, category membership is a matter of degree. In the domain of child psychopathology this means that problem behaviors can be more or less typical for a particular prototype. According to Chan and Jackson (1982), intuitive prototypes or implicit personality theories arise from the observation of co-occurrences of behaviors. Therefore, they reflect the distribution of traits in people and provide a relatively valid foundation for the judgement of specific targets.The issue of whether these intuitive prototypes reflect reality has been raised (