OBJECTIVE: Observational studies have reported inverse associations between adherence to the Mediterranean diet (MedDiet) and atherosclerotic disease. We tested the effect of two types of MedDiet on progression of subclinical carotid atherosclerosis. METHODS: We randomized 187 high-cardiovascular-risk asymptomatic subjects (51% women, mean age 67 years) to three treatment arms: MedDiet with supplemental virgin olive oil (VOO), n=66; MedDiet with supplemental nuts, n=59; and control diet, n=62. Participants received nutrition behavioral counseling in quarterly group and individual educational sessions. Free supplemental foods were provided to the MedDiet groups. Changes in mean intima-media thickness (IMT) were measured ultrasonographically in the far wall of bilateral common carotid arteries after 1 year. RESULTS: Overall, no significant between-group differences in IMT progression were observed after 1-year. However, a significant interaction (p=0.03) between baseline IMT and treatment effect was apparent. Among participants with baseline IMT≥0.9 mm, 1-year IMT changes versus control showed significant differences of -0.079 mm (95% confidence interval, -0.145 to -0.012) for the MedDiet with VOO and -0.072 mm (-0.140 to -0.004) for the MedDiet with nuts. No IMT changes occurred in any intervention group among participants with lower baseline IMT values (<0.9 mm). CONCLUSIONS: MedDiets enhanced with VOO or nuts were not effective in inducing ultrasonographic regression of carotid atherosclerosis after 1 year intervention. However, they were effective among subjects with elevated baseline IMT, suggesting that subclinical atherosclerosis may respond to dietary intervention within a relatively short time frame only among subjects with a high initial atherosclerotic burden.