The Grade of Membership (GoM) model is a classification procedure which allows a person to be a member of more than one diagnostic class. It simultaneously quantifies the degrees of membership in classes while generating the discrete symptom profiles or 'pure types' describing classes. The model was applied to the symptomatology, history, and follow-up of 1065 cases in the WHO International Pilot Study of Schizophrenia. The model produced an eight diagnostic class or 'pure type' solution, of which five were related to the diagnostic concepts of schizophrenia and paranoid disorder, two types were affective disorders, and one asymptomatic type. A subtype of paranoid schizophreniform disorder found primarily in developing countries was identified. There was a strong association between pure types and the original clinical and computer generated (CATEGO) diagnoses. A GoM based psychiatric classification might more clearly identify core disease processes than conventional classification models by filtering the confounding effects of individual heterogeneity from pure type definitions.