Context Higher adherence to a Mediterranean-type diet is linked to lower risk for mortality and chronic diseases, but its association with cognitive decline is unclear. Objective To investigate the association of a Mediterranean diet with change in cognitive performance and risk for dementia in elderly French persons. Design, Setting, and Participants Prospective cohort study of 1410 adults (>= 65 years) from Bordeaux, France, included in the Three-City cohort in 20012002 and reexamined at least once over 5 years. Adherence to a Mediterranean diet ( scored as 0 to 9) was computed from a food frequency questionnaire and 24-hour recall. Main Outcome Measures Cognitive performance was assessed on 4 neuropsychological tests: the Mini-Mental State Examination (MMSE), Isaacs Set Test (IST), Benton Visual Retention Test (BVRT), and Free and Cued Selective Reminding Test (FCSRT). Incident cases of dementia (n=99) were validated by an independent expert committee of neurologists. Results Adjusting for age, sex, education, marital status, energy intake, physical activity, depressive symptomatology, taking 5 medications/d or more, apolipoprotein E genotype, cardiovascular risk factors, and stroke, higher Mediterranean diet score was associated with fewer MMSE errors (beta=-0.006; 95% confidence interval [CI], -0.01 to -0.0003; P=.04 for 1 point of the Mediterranean diet score). Performance on the IST, BVRT, or FCSRT over time was not significantly associated with Mediterranean diet adherence. Greater adherence as a categorical variable (score 6-9) was not significantly associated with fewer MMSE errors and better FCSRT scores in the entire cohort, but among individuals who remained free from dementia over 5 years, the association for the highest compared with the lowest group was significant (adjusted for all factors, for MMSE: beta=-0.03; 95% CI, -0.05 to -0.001; P=.04; for FCSRT: beta=0.21; 95% CI, 0.008 to 0.41; P=. 04). Mediterranean diet adherence was not associated with the risk for incident dementia (fully adjusted model: hazard ratio, 1.12; 95% CI, 0.60 to 2.10; P=.72), although power to detect a difference was limited. Conclusions Higher adherence to a Mediterranean diet was associated with slower MMSE cognitive decline but not consistently with other cognitive tests. Higher adherence was not associated with risk for incident dementia. JAMA. 2009; 302(6):638-648 www.jama.com