Objectives To our knowledge, no study has addressed the relationship between adherence to a Mediterranean diet style and severity of heart failure. The aim of this retrospective study was to evaluate the relationship between adherence to the Mediterranean diet assessed using the calculation of Mediterranean diet score and congestive heart failure (CHF), its severity, and pathogenesis. Methods We analyzed charts and collected data of all consecutive patients with a diagnosis of CHF at admission to our Internal Medicine Ward from 2008 to 2014. Results We analyzed 209 patients with CHF and 200 controls. Patients with CHF showed a significantly lower mean MeDi score than controls. At receiver operating characteristic curve analysis, we found a good sensitivity and specificity of mean MeDi score to predict CHF. We also observed a significant positive correlation between MeDi score and ischemic pathogenesis of CHF, a positive relationship between New York Heart Association (NYHA) class and ischemic heart disease, and a significant negative relationship between NYHA class and MeDi score. Conclusion The beneficial effects of adherence to the Mediterranean diet suggest a possible answer to the question of the biochemical bases of our data, which should be seen as the direct consequence of the anti-inflammatory, antioxidant, and anti-remodeling effects linked to the diet.