OBJECTIVES: Apolipoprotein (Apo)B, ApoA-I, and their ratio could predict coronary heart disease (CHD) risk more accurately than conventional lipid measurements. Our aim was to assess the effect of a traditional Mediterranean diet (TMD) on apolipoproteins. METHODS: High-cardiovascular risk subjects (n=551, 308 women and 243 men), aged 55-80 years, were recruited into a large, multicenter, randomized, controlled, parallel-group, clinical trial (The PREDIMED Study) aimed at testing the efficacy of TMD on primary cardiovascular disease prevention. Participants assigned to a low-fat diet (control) (n=177), or TMDs (TMD+virgin olive oil (VOO), n=181 or TMD+nuts, n=193) received nutritional education and either free VOO (ad libitum) or nuts (dose: 30 g/day). A 3-month evaluation was performed. RESULTS: Both TMDs promoted beneficial changes on classical cardiovascular risk factors. ApoA-I increased, and ApoB and ApoB/ApoA-I ratio decreased after TMD+VOO, the changes promoting a lower cardiometabolic risk. Changes in TMD+VOO versus low-fat diet were -2.9 mg/dL (95% CI, -5.6 to -0.08), 3.3mg/dL (95% CI, 0.84 to 5.8), and -0.03 mg/dL (-0.05 to -0.01) for ApoB, ApoA-I, and ApoB/ApoA-I ratio, respectively. CONCLUSIONS: Individuals at high-cardiovascular risk who improved their diet toward a TMD pattern rich in virgin olive oil, reduced their Apo B and ApoB/ApoA-I ratio and improved ApoA-I concentrations.