The nature of the association between IQ and schizophrenia is still unclear. So far no study addressed this issue in relation to the breadth or scope of the very concept of schizophrenia. We examined the premorbid IQ in a polydiagnostic study with four classifications of schizophrenia: ICD-8/9, ICD-10, St. Louis and Flexible System-Wide. Only the ICD-10 schizophrenia patients exhibited a significantly lower premorbid IQ. There were suggestive differences between the four examined systems as well as between the ICD-10 paranoid and non-paranoid subtypes. Exploration of crucial diagnostic features of schizophrenia in relation to IQ revealed associations between low premorbid IQ and hallucinations as well as negative symptoms. It is concluded that premorbid IQ varies across different definitions of schizophrenia.