The limitations of establishing neuropsychological diagnosis through psychological testing and psychiatric examination by clinicians are discussed, along with the need to define cases of illness in a standardized way for research. The GMS-HAS-AGECAT package is a standardized assessment of mental state and historical information about onset of illness, from which data are used in a computer-assisted method to derive clinically based diagnoses of the principal types of dementia, depression and other mental illness. Recording is made of co-morbid states and levels of diagnostic confidence. Agreement between psychiatrists on AECAT diagnoses ranges from kappa values of 0.76 to 0.88, and validity has now been tested further by outcome studies. A short description is given of the Medical Research Council, EURODEM, World Health Organization and Pan American Health Organization studies now using this method.