The Geriatric Mental State Examination (GMS), a standardised psychiatric interview, and its computerised diagnostic system, AGECAT, have been applied to a large (1070) sample of subjects aged over 65 in Liverpool. In a split-half study of this sample, diagnostic scales within the GMS and a diagnostic index derived from them were tested for efficacy in cases of organic and early organic illness (mostly various stages of dementia) and depressive illness. Results demonstrated high levels of sensitivity and specificity, and positive predictive values were within the expected range when applied to these disorders, given their low prevalence in the community. The GMS, when used in this way, is a flexible and effective case-finding instrument.