OBJECTIVE: We tested the hypothesis that impaired kidney function in the elderly is associated with a more rapid rate of cognitive decline. METHODS: Baseline serum was used to calculate estimated glomerular filtration rate (eGFR), using the Modification of Diet in Renal Disease formula, for 886 elderly without dementia participating in the Rush Memory and Aging Project, a prospective, observational cohort study. Kidney function was also dichotomized into impairment or no impairment based on eGFR < or ? 60 mL/min/1.73 m. Structured cognitive testing was performed at baseline and at annual evaluations, using a battery of 19 cognitive tests summarized into global cognition and 5 cognitive domains. RESULTS: In mixed-effects models adjusted for age, sex, and education, a lower eGFR at baseline was associated with a more rapid rate of cognitive decline (estimate 0.0008, SE <0.001, p = 0.017). The increased rate of cognitive decline associated with a 15-mL/min/1.73 m lower eGFR at baseline (approximately 1 SD) was similar to the effect of being 3 years older at baseline. Impaired kidney function at baseline was associated with a more rapid rate of cognitive decline (estimate-0.028, SE <0.009, p = 0.003). The increased rate of cognitive decline associated with impaired kidney function at baseline was approximately 75% the effect of ApoE4 allele on the rate of cognitive decline. Baseline kidney function was associated with declines in semantic memory, episodic memory, and working memory but not visuospatial abilities or perceptual speed. CONCLUSION: Impaired kidney function is associated with a more rapid rate of cognitive decline in old age.