Objective: The aim of the study was to examine the relationship between longitudinally assessed cognitive functioning and self-reported dementia status using the Ascertain Dementia 8-item questionnaire (AD8) in a national population-based sample. Methods: The analysis included 14,453 participants from the REasons for Geographic and Racial Differences in Stroke study. A validated cutoff of ≥2 symptoms endorsed on the AD8 (administered 10 years after enrollment) represented positive AD8 status. Incident cognitive impairment was defined as change from intact to impaired status in the Six-Item Screener score, and cognitive decline was defined by trajectories of Letter "F"Fluency from the Montreal Cognitive Assessment, and Animal Fluency, Word List Learning, and Word List Delayed recall, all from the Consortium to Establish a Registry for Alzheimer's Disease battery. Logistic regression models controlled for demographics, health variables, and depressive symptoms. Results: Sensitivity and specificity of the AD8 to detect incident cognitive impairment were 45.2% and 78.4%, respectively. Incident cognitive impairment and a one-word decline in WLL increased the odds of self-reported positive AD8 by 96% (95% CI: 1.68-2.28) and 27% (95% CI: 1.17-1.37), respectively. There was a strong association between high depression risk and self-reported positive AD8 in sensitivity analyses. Conclusions: Incident cognitive impairment and high depression risk were the strongest predictors of self-reported positive AD8 in this population-based sample. Our results inform the utility of the AD8 as a self-report measure in a large, national sample that avoids selection biases inherent in clinic-based studies. The AD8 is screening measure and should not be used to diagnose dementia clinically.