In the modern classification systems ICD-10 and DSM-III-R, the term seasonality has been given a separate significance in the diagnostic categorization of affective disorders. Its definition is oriented not merely to the increased seasonal prevalence in the autumn and winter months, but also to symptoms that do not occur in nonseasonal attacks. To date, research work on seasonality in schizophrenia has concentrated on birth seasonality, while no results based on large populations of patients have yet become available on the manifestation seasonality of schizophrenic subtypes. Within the framework of a retrospective study carried out in the period between 1983 and 1995, involving 2,119 patients suffering from recurrent attacks of schizophrenia, seasonal manifestations were recorded in accordance with the criteria defined in DSM-III-R. It was found that the two subtypes of paranoid hallucinatory and schizoaffective psychoses met the criterion of seasonality significantly more frequently than did the subtypes disorganized and catatonic disorders. In addition, seasonal courses within the groups just mentioned clearly differed from the nonseasonal forms in terms of their symptomatology.