The study's aims were to empirically derive classes of disorders and dimensional syndromes within psychotic disorders on the basis of the three time frames of symptom assessment and to comparatively examine their external validity. The level of concordance among classes and among dimensions across the time frames was generally low. The external correlates of psychopathological syndromes differed as a function of both type of assessment and the dimensional or categorical approach used. The dimensional approach was more effective than the categorical approach in predicting a set of clinical variables, irrespective of the time frame used to assess the symptoms. It is concluded that classification of psychotic disorders is highly dependent upon the time frame considered to assess symptoms and that dimensional classifications do have higher predictive power than categorical ones.