Diet quality has not been well studied in relation to positive psychological traits. Our purpose was to investigate the relationship between optimism and diet quality in postmenopausal women enrolled in the Women's Health Initiative observational study (OS) and clinical trials (CTs), and to determine whether optimism was associated with diet change after a 1-year dietary intervention. Diet quality was scored with the Alternate Healthy Eating Index (AHEI) and optimism assessed with the Life Orientation Test-Revised. Baseline characteristics were compared across AHEI quintiles or optimism tertiles using regression models with each variable of interest as a function of quintiles or tertiles (OS, n=87,630; CT, n=65,360). Association between optimism and baseline AHEI and change in AHEI over 1 year were tested using multivariate linear regression (CT, n=13,645). Potential interaction between optimism and trial arm and demographic/lifestyle factors on AHEI change was tested using likelihood ratio test (CT intervention, n=13,645; CT control, n=20,242). Women reporting high AHEI were non-Hispanic white, educated, physically active, past or never smokers, hormone therapy users, had lower body mass index and waist circumference, and were less likely to have chronic conditions. In the CT intervention, higher optimism was associated with higher AHEI at baseline and with greater change over 1 year (P=0.001). Effect modification by intervention status was observed (P=0.014), whereas control participants with highest optimism achieved threefold greater AHEI increase compared with those with the lowest optimism. These data support a relationship between optimism and dietary quality score in postmenopausal women at baseline and over 1 year.