['Little evidence exists on change in diet quality and weight change.', 'We examined the association between change of diet quality indexes and concurrent weight change over 20 y.', "In this analysis we followed 50,603 women in the Nurses' Health Study (NHS), 22,973 men in the Health Professionals Follow-Up Study (HPFS) between 1986 and 2006, and 72,495 younger women from the Nurses' Health Study II (NHS II) between 1989 and 2007. Diet was measured every 4 y. We computed the Alternate Mediterranean Diet, the Alternate Health Eating Index-2010 (AHEI-2010), and the Dietary Approaches to Stop Hypertension adherence scores for each participant. All scores emphasize fruits and vegetables, whole grains, and nuts, but they differ in score range and components such as dairy, sodium, and sweetened beverages. Regression models were used to examine 4-y changes in these scores and weight change within the same period, adjusting for lifestyle factors.", 'Mean age at baseline was 49.4 y for NHS, 48.0 y for HPFS, and 36.3 y for NHS II. Baseline BMI (in kg/m(2)) was similar (23.7 for NHS, 24.7 for HPFS, and 23.0 for NHS II). We observed significantly less weight gain over 4-y periods with each SD increase of each diet quality score in both men and women. Results were significantly stronger in the younger cohort (NHS II) than in the older cohorts (e.g., -0.67 kg less weight gain in NHS II vs. -0.39 kg in NHS for each SD increase in AHEI-2010; P-heterogeneity: <0.001). Improvement of any of the diet scores benefited overweight (-0.27 to -1.08 kg less weight gain for each SD increase in score) more than normal-weight individuals (-0.10 to -0.40 kg; P-interaction: <0.001).'] Does long-Term Change in Diet Quality be Associated with Body Weight Change in Men and Women?