Concern has been expressed as to the reliability of clinical ICD-10 diagnosis of schizophrenia. This study was designed to assess the diagnostic reliability of the clinical ICD-10 diagnosis of schizophrenia in a random sample of Danish in- and outpatients with a history of psychosis. A sample of 100 subjects was assessed using the operational criteria OPCRIT checklist for psychotic and affective illness. The most recent principal and clinical ICD-10 diagnosis was compared with diagnoses generated by the OPCRIT instrument. Data documented very high sensitivity (93%) and positive predictive value (87%) of ICD-10 schizophrenia and an overall good agreement between clinical and OPCRIT-derived diagnoses (kappa=0.60). An even higher positive predictive value was obtained when diagnoses were amalgamated into a diagnostic entity of schizophrenia-spectrum disorders (98%). Near perfect agreement was seen between OPCRIT-derived ICD-10 and DSM-IV diagnoses (kappa=0.87). Thus, this study demonstrates high reliability of the clinical diagnosis of schizophrenia and even more so of the diagnosis of schizophrenia-spectrum disorder.