Background: The aim was to examine the agreement and differences between ICD-10 and DSM-IV in the classification of functional psychoses. Sampling and Methods: In a sample of 218 first-hospitalised patients, ICD-10 diagnoses were compared with DSM-IV diagnoses. Functional psychoses of both diagnostic systems were classified into the four diagnostic groups schizophrenia, transient/episodic psychoses, delusional disorders and affective disorders. Based on information from a 15-year follow-up, it was examined which course is associated with each diagnostic group. Results: Although in ICD-10 there was a higher frequency of schizophrenia and a lower one of affective disorders, a high agreement between ICD-10 and DSM-IV (kappa value of 0.82) was found. In both diagnostic systems, transient/episodic psychoses and affective disorders were mainly associated with a non-chronic course and schizophrenia was mainly associated with a chronic one. Nevertheless, several patients with transient/episodic psychoses showed a chronic course (ICD-10: 10%, DSM-IV: 15%) and more than one third of patients with schizophrenia a non-chronic one (ICD-10: 40%, DSM-IV: 33%). Conclusions: In the cross-sectional assessment, there is a high diagnostic agreement between ICD-10 and DSM-IV. With respect to the long-term course, the delimitation of transient/episodic psychoses from schizophrenia was neither completely achieved by ICD-10 nor by DSM-IV. Copyright (C) 2004 S. Karger AG, Basel.