Increased susceptibility of the aging brain to both chronic stress and incipient dementia-related neuropathology may accelerate cognitive decline. We investigated associations between chronic stress and diagnostic change in 62 individuals (mean age, 78.7 y) participating in an Alzheimer disease research center longitudinal study. The subjects, diagnosed at baseline as cognitively normal (CN) or with mild cognitive impairment (MCI), were followed for an average of 2.5 years. Senior neurologists, blind to detailed measures of stress and cognition, assigned diagnoses annually. Logistic regression analyses assessed the accuracy with which measures of stress (event-based ratings, cortisol levels) predicted the conversion to MCI and dementia. Eleven individuals with MCI at baseline received a dementia diagnosis during follow-up. Sixteen converted from cognitively normal to MCI. Prolonged, highly stressful experiences were associated with conversion from MCI to dementia. The cortisol awakening response, with age and education, was associated with a diagnostic change to MCI. Cortisol measures were not associated with the progression from MCI to dementia, and there was no association between stressful experiences and the change to MCI. Mechanisms associated with the transition from normal cognition to MCI may differ from those associated with a diagnostic change to dementia. These findings could facilitate the identification of interventional strategies to reduce the risk of decline at different stages of susceptibility.