A series of in-patients, a series of out-patients and a series of women from a general population sample, all aged under 65, and all living in the same area of south-east London, were examined using the ninth edition of the Present State Examination (PSE). An 'index of definition' (ID), based on number, type and severity of PSE symptoms, was constructed in order to define a threshold point above which sufficient information was available to allow classification into one of the functional psychoses or neuroses. The index specified 8 levels of definition of disorder; the threshold point for a 'case' lying between levels 4 and 5. The identification of 'cases' by this means in the general population series was highly concordant with global clinical judgement. When the 3 series were compared, the in-patient series contained the most definite cases, the outpatient series was intermediate, and the general population series was characterized mainly by below-threshold conditions. Disorders above the threshold point were classified using the CATEGO program. The clinical severity of the depressive disorders and anxiety states identified in this way in the 3 series was compared on the basis of symptom and syndrome profiles and PSE scores. As expected, the inpatients had the most severe, and the general population 'cases' the least severe disorders. It is suggested that the PSE-ID-CATEGO techniques can be used to help standardize certain aspects of case-finding and case-description in both referred and non-referred populations and thereby make the results of surveys more comparable.