Objectives: To examine the earliest cognitive changes associated with the onset of dementia as well as changes associated with normal aging. Design: Longitudinal evaluation of participants with annual clinical and psychometric examinations for up to 15 1/4 years. Setting and Participants: Elderly volunteers (n=82) enrolled with a Clinical Dementia Rating of 0 (cognitively intact) in longitudinal studies. Interventions: None. Main Outcome Measures: Clinical Dementia Rating and results of a 1 1/4 -hour psychometric battery. Results: As estimated with survival analysis, 40% of participants had a Clinical Dementia Rating greater than 0 (cognitive decline) within 12 years of enrollment; 59% of these were judged to have dementia of the Alzheimer type or incipient dementia. Participants with poorer performance on psychometric testing at enrollment were at higher risk for cognitive decline subsequently. The rate of change in psychometric performance before clinically detectable cognitive change occurred was not significantly different between those who eventually developed dementia and those who remained stable, except for performance on the Logical Memory subtest of the Wechsler Memory Scale. When subtle cognitive decline was clinically detected, however, an abrupt deterioration in performance on independently administered psychometric tests was observed. Conclusions: Cognitively healthy elderly people maintain stable cognitive performance when measured longitudinally by both careful clinical evaluation and repeated psychometric testing. This stability is maintained unless and until they develop a dementing illness, at which time a sharp decline in performance is observed.