The compatibility of the classification systems ICD-10 and DSM-III-R was investigated in a sample of 100 psychiatric inpatients with severe affective and psychotic disorders. Four independent raters assessed diagnoses by means of checklists from psychopathological descriptions of case records. Congruence between ICD-10 and DSM-III-R was good for depressive disorders with agreement rates above 80% and fair for bipolar disorders with rates above 60%. Less consistent findings were obtained for schizophrenia with rates of 57.5% for ICD-10 diagnoses and 82.6% for DSM-III-R diagnoses. Agreement for schizoaffective disorder was below 40% and thus insufficient. The results indicate that even small differences in the definitions of mental disorders may result in considerable inconsistencies.