BACKGROUND: Epidemiologic observations have linked healthy dietary patterns to improved kidney function. STUDY DESIGN: We assessed the effects of the Mediterranean diet (MedDiet) on kidney function in both a cross-sectional assessment and after a 1-year intervention in a cohort of the PREDIMED (Prevención con Dieta Mediterránea) Study, a multicenter 3-arm randomized clinical trial to determine the efficacy of the MedDiet on primary cardiovascular prevention. SETTING & PARTICIPANTS: Community-dwelling men aged 55-80 years and women aged 60-80 years at high risk of cardiovascular disease from Reus, Spain. INTERVENTION: Participants were randomly assigned to 3 ad libitum diets: a MedDiet supplemented with virgin olive oil (MedDiet + olive oil), a MedDiet supplemented with mixed nuts (MedDiet + nuts), or a control low-fat diet. OUTCOMES: Estimated glomerular filtration rate (eGFR) and urinary albumin-creatinine ratio (ACR). MEASUREMENTS: Nutrient intake, adherence to the MedDiet, lifestyle variables, cardiovascular risk factors, serum urea and creatinine concentrations, eGFR, and urinary ACR were evaluated at baseline and after intervention for 1 year. RESULTS: Baseline kidney function markers were similar across quartiles of adherence to the MedDiet in 785 participants (55% women; mean age, 67 years). After a 1-year intervention in 665 participants, the 3 dietary approaches were associated with improved kidney function, with similar average increases in eGFR (4.7 [95% CI, 3.2-6.2], 3.5 [95% CI, 1.9-5.0], and 4.1 [95% CI, 2.8-5.5] mL/min/1.73 m(2) for the MedDiet + olive oil, MedDiet + nuts, and control groups, respectively [P < 0.001 vs baseline for each; P = 0.9 for differences among groups]), but no changes in ACRs after adjustment for various confounders. LIMITATIONS: Generalization of results to other age groups or ethnicities. GFR was not directly measured. CONCLUSIONS: The results do not support the notion that the MedDiet has a beneficial effect on kidney function over and above that of advice for a low-fat diet in elderly individuals at high cardiovascular risk.