Clinical investigations of senile dementia of the Alzheimer type require establishment of explicit clinical diagnostic criteria before histological confirmation is possible. Criteria for selection of mildly impaired subjects with senile dementia of Alzheimer type, free of other major disease, are proposed. Problems of recruitment of this select population for a longitudinal study are discussed. A study population with matched healthy control subjects has been enrolled and described. Short term follow-up has provided preliminary support for the diagnostic criteria.