BACKGROUND: Interventions promoting weight loss can reduce the incidence of type 2 diabetes mellitus. Whether dietary changes without calorie restriction also protect from diabetes has not been evaluated. OBJECTIVE: To assess the efficacy of Mediterranean diets for the primary prevention of diabetes in the Prevención con Dieta Mediterránea trial, from October 2003 to December 2010 (median follow-up, 4.1 years). DESIGN: Subgroup analysis of a multicenter, randomized trial. (Current Controlled Trials: ISRCTN35739639) SETTING: Primary care centers in Spain. PARTICIPANTS: Men and women without diabetes (3541 patients aged 55 to 80 years) at high cardiovascular risk. INTERVENTION: Participants were randomly assigned and stratified by site, sex, and age but not diabetes status to receive 1 of 3 diets: Mediterranean diet supplemented with extra-virgin olive oil (EVOO), Mediterranean diet supplemented with nuts, or a control diet (advice on a low-fat diet). No intervention to increase physical activity or lose weight was included. MEASUREMENTS: Incidence of new-onset type 2 diabetes mellitus (prespecified secondary outcome). RESULTS: During follow-up, 80, 92, and 101 new-onset cases of diabetes occurred in the Mediterranean diet supplemented with EVOO, Mediterranean diet supplemented with mixed nuts, and control diet groups, respectively, corresponding to rates of 16.0, 18.7, and 23.6 cases per 1000 person-years. Multivariate-adjusted hazard ratios were 0.60 (95% CI, 0.43 to 0.85) for the Mediterranean diet supplemented with EVOO and 0.82 (CI, 0.61 to 1.10) for the Mediterranean diet supplemented with nuts compared with the control diet. LIMITATIONS: Randomization was not stratified by diabetes status. Withdrawals were greater in the control group. CONCLUSION: A Mediterranean diet enriched with EVOO but without energy restrictions reduced diabetes risk among persons with high cardiovascular risk. PRIMARY FUNDING SOURCE: Instituto de Salud Carlos III.