The dietary modification trial component studies the effect of a low-fat, high fruit, vegetable and grain diet on breast cancer, colorectal cancer and heart disease in 48,000 postmenopausal women. Participants are randomized to a comparison group which maintains usual dietary habits or to a dietary change group. Women in the dietary change group decrease their fat intake to 20 percent of total daily calories, increase fruit and vegetable consumption to five or more servings per day, and increase grains to six or more servings per day. Additionally, they monitor their food intake and attend nutrition group meetings to learn more about changing their diets in the first year. Thereafter they attend four meetings per year.