BACKGROUND: The association between diet quality and risk of incident cardiovascular disease (CVD) or heart failure (HF) in postmenopausal women is uncertain. OBJECTIVE: This study aimed to determine whether a conventional index [Alternate Healthy Eating Index (AHEI)] or a novel index [Women's Health Initiative (WHI) Dietary Modification Index (DMI)] of diet quality was associated with the risk of incident CVD or HF in the WHI Observational Study (WHI-OS). DESIGN: The WHI-OS is an observational cohort study of 93,676 women aged 50-79 y of diverse ethnicity and backgrounds followed for an average of 10.0 y for CVD events. The individual components of the AHEI and DMI were determined from the baseline WHI food-frequency questionnaire. Incident CVD was a composite of nonfatal myocardial infarction, coronary heart disease death, stroke, coronary revascularization, and incident HF. The association between AHEI or DMI and incident CVD or incident HF was determined by using Cox models adjusted for traditional CVD and HF risk factors. RESULTS: Women with a DMI in the highest quintile had hazard ratios (HRs) of 0.88 (95% CI: 0.80, 0.95) and 0.91 (95% CI: 0.78, 1.06) for incident CVD and HF, respectively. Women with an AHEI in the highest quintile had HRs of 0.77 (95% CI: 0.70, 0.84) and 0.70 (95% CI: 0.59, 0.82) for incident CVD and HF, respectively. CONCLUSION: Overall, adherence to current nutrient guidelines (as indexed by the DMI) are associated with lower total CVD risk, and additional dietary factors (as indexed by the AHEI) were associated with a lower risk of CVD and HF.