This article presents a review of some literature on paranoid schizophrenia from the viewpoints of diagnostic classification and biological research. With regard to classification there is an increasing tendency to assign a large number of the 'Bleulerian' paranoid schizophrenics to the groups of affective and schizo-affective disease. From the biological point of view this tendency seems justified. In fact many findings suggest that groups of paranoid schizophrenics differ from other schizophrenic subgroups in possessing physical, physiological or biochemical peculiarities which are found in affective psychosis as well. Research criteria for paranoid schizophrenia should include

• diagnosis of schizophrenia only after exclusion of affective disorder,
• operationalisation of age at onset (e.g. between 24 and 40 years),
• a chronic course of illness,
• absence of periodical excitement and catatonic signs,
• repeated or continuous predomination of delusions or hallucinations.