ABSTRACT Background: Chronic inflammation is a central mechanism involved in cardiovascular diseases, cancer, diabetes, and chronic respiratory diseases, 4 leading causes of mortality. Diet is a major source of pro- and anti-inflammatory bioactive compounds. The Dietary Inflammatory Index (DII) was designed to estimate the overall inflammatory potential of the diet. Objective: Our aim was to study the prospective association between the DII and mortality, as well as assess whether antioxidant supplementation could modulate this association. Design: The Supplémentation en Vitamines et Minéraux Antioxydants study was a randomized, double-blind, placebo-controlled trial in which participants received low-dose antioxidants or a placebo from 1994 to 2002. In this observational prospective analysis, 8089 participants (mean ± SD age at baseline: 49.0 ± 6.3 y) were followed between 1994 and 2007 (median: 12.4 y). The DII was calculated from repeated 24-h dietary records; higher scores correspond to more proinflammatory diets. A total of 207 deaths occurred during follow-up, including 123 due to cancer and 41 due to cardiovascular events. Multivariate Cox proportional hazards models were computed. Results: Sex-specific tertiles of the DII were positively associated with cardiovascular + cancer mortality (HR for tertile 3 compared with tertile 1 = 1.53; 95% CI: 1.01, 2.32; P-trend = 0.05) and specific cancer mortality (HR for tertile 3 compared with tertile 1 = 1.83; 95% CI: 1.12, 2.99; P-trend = 0.02). The corresponding P value was 0.07 for all-cause mortality. The DII was statistically significantly associated with increased all-cause mortality in the placebo group (HR for tertile 3 compared with tertile 1 = 2.10; 95% CI: 1.15, 3.84; P-trend = 0.02) but not in the antioxidant-supplemented group (P-trend = 0.8; P-interaction = 0.098). Conclusion: These results suggest that a proinflammatory diet is associated with increased all-cause and cancer mortality and antioxidants may counteract some of the proinflammatory effects of the diet. This trial was registered at clinicaltrials.gov as NCT00272428.