In Scotland the Information and Statistics Division of the Scottish Office Home and Health Department maintains a computerised record of patients who have been admitted to, discharged from or have died in Scottish mental hospitals. This data source is of particular value to those involved in planning health care provision and to epidemiologists. The present study assesses the quality of these data in relation to patients with verified presenile dementia who have been discharged from the Royal Edinburgh Hospital between 1974 and 1988. Seventeen per cent of presenile patients were not identified and 23% of patients were wrongly classified. More than 93% of patients with presenile Alzheimer's disease were identified, though often under a variety of other diagnostic codes. These findings highlight limitations of the data supplied and suggest that the usefulness of this enviable and unique data source could be enhanced if the medical profession took greater care in clearly stating an International Classification of Diseases diagnosis in a patient's hospital record.