By comparing six different operational diagnostic systems (International Classification of Diseases, Diagnostic and Statistical Manual; 3rd ed., Research Diagnostic Criteria, St. Louis criteria, Taylor criteria and Vienna Research Criteria), the data presented in this paper illustrate how attribution to various categories of functional psychoses varies according to the applied algorithms. Bleuler’s basic symptoms are obviously considered by all of the compared systems to be more significant for attribution to schizophrenia than first-rank symptoms.