A modified version of the revised Personality Diagnostic Questionnaire (PDQ-R), based on DSM-III-R personality disorders (PDs), was completed by 60 psychiatric patients. An informant's version was also completed by 60 relatives or friends nominated by each subject. Discrete DSM-III-R PDs were rare; the mean number of PDs per subject was 4.5. Cluster analysis showed that only antisocial PD was a basis for classification of patients, while most patients formed two groups which were mainly distinguished by quantitative differences related to the total scores of positive PD criteria. A shorter version of the questionnaire can be used as a screening test for co-morbid PDs (STCPD) which can predict the number of co-morbid DSM-III-R PDs. The total scores of positive PD criteria from the STCPD were usually (and significantly) higher than the corresponding scores from informants' questionnaires, but when an informant's total score exceeded that of the patient, this indicated a subject's under-reporting.