Abstract Aims Scarce data are available on the effect of the traditional Mediterranean diet (TMD) on heart failure biomarkers. We assessed the effect of TMD on biomarkers related to heart failure in a high cardiovascular disease risk population. Methods and Results A total of 930 subjects at high cardiovascular risk (420 men and 510 women) were recruited in the framework of a multicentre, randomized, controlled, parallel-group clinical trial directed at testing the efficacy of the TMD on the primary prevention of cardiovascular disease (The PREDIMED Study). Participants were assigned to a low-fat diet (control, n = 310) or one of two TMDs [TMD + virgin olive oil (VOO) or TMD + nuts]. Depending on group assignment, participants received free provision of extra-virgin olive oil, mixed nuts, or small non-food gifts. After 1 year of intervention, both TMDs decreased plasma N-terminal pro-brain natriuretic peptide, with changes reaching significance vs. control group (P < 0.05). Oxidized low-density lipoprotein decreased in both TMD groups (P < 0.05), the decrease in TMD + VOO group reaching significance vs. changes in control group (P = 0.003). Changes in lipoprotein(a) after TMD + VOO were less than those in the control group (P = 0.046) in which an increase (P = 0.035) was observed. No changes were observed in urinary albumin or albumin/creatinine ratio. Conclusions Individuals at high risk of cardiovascular disease (CVD) who improved their diet toward a TMD pattern reduced their N-terminal pro-brain natriuretic peptide compared with those assigned to a low-fat diet. The same was found for in vivo oxidized low-density lipoprotein and lipoprotein(a) plasma concentrations after the TMD + VOO diet. From our results TMD could be a useful tool to mitigate against risk factors for heart failure. From our results TMD could modify markers of heart failure towards a more protective mode.