The aim of this study was to investigate the concepts of reactive and hysterical psychoses and how they are classified in standardized diagnostic systems. To this end we identified all of the patients who had been admitted to a psychiatric in-patient unit and diagnosed as suffering from psychogenic psychosis, reactive psychosis, hysterical psychosis or hysteria, using ICD-9 criteria. The case notes of these patients were then re-examined and diagnoses reached using DSM-III-R, DSM-IV and ICD-10 criteria and the Present State Examination (PSE)/CATEGO computer program. The objective of this study was to evaluate the agreement between the diagnoses of reactive and hysterical psychosis obtained using ICD-9 criteria with those obtained using the DSM-III-R, DSM-IV, ICD-10 and PSE diagnostic systems. A total of 67 case notes were identified in which the above diagnoses had been made: 27 cases with ICD-9 'hysteria' and 26 cases with 'other reactive and not otherwise specified psychoses'. Using the DSM-III-R criteria, 27 cases were diagnosed as psychotic disorder NOS, 12 as brief reactive psychosis and 11 as bipolar disorder. Using the DSM-IV criteria, 21 cases were diagnosed as psychotic disorder NOS, 11 as mood disorder, 7 as brief disorder without stressor, and 12 as brief disorder with stressor. Using the ICD-10 criteria, 18 cases were diagnosed as unspecified non-organic psychosis, 12 as mood disorder, 10 as acute and transient psychotic disorder without stressor and 13 as acute and transient psychotic disorder with stressor. Using the PSE/CATEGO program, the most common diagnoses were class 'S' schizophrenia (17), class 'P?' uncertain psychosis (16) and class 'M+' mixed and manic affective disorder (11). Using the kappa coefficient a very low level of agreement was found between ICD-9 'hysteria' and 'other reactive and non-specified psychoses' and the corresponding categories of DSM-III-R and the PSE/CATEGO program. We concluded that, although DSM-III-R provides operational criteria for brief reactive psychosis, and DSM-IV and ICD-10 provide such criteria for brief or acute psychotic disorder, these bear little relationship to the original concept of the disorder. The PSE/CATEGO program provides a very systematic approach to symptomatology, but the diagnostic classes have little clinical usefulness.