Background: Both portion size and energy density (ED) have substantial effects on intake; however, their combined effects on preschool children's intake have not been examined when multiple foods are varied at a meal. Objective: We tested the effects on intake of varying the portion size and ED of lunches served to children in their usual eating environment. Design: In a crossover design, lunch was served in 3 childcare centers once a week for 6 weeks to 120 children aged 3-5 y. Across the 6 meals, all items were served at 3 levels of portion size (100%, 150%, or 200%) and 2 levels of ED (100% or 142%). The lunch menu had either lower-ED or higher-ED versions of chicken, macaroni and cheese, vegetables, applesauce, ketchup, and milk. Children's ratings of the foods indicated that the lower-ED and higher-ED meals were similarly well liked. Results: The total weight of food and milk consumed at meals was increased by serving larger portions (P less than 0.0001) but was unaffected by varying the ED (P equal to 0.22).Meal energy intake, however, was independently affected by portion size and ED (both P less than 0.0001). Doubling the portions increased energy intake by 24% and increasing meal ED by 42% increased energy intake by 40%. These effects combined to increase intake by 175 plus or minus 12 kcal or 79% at the higher-ED meal with the largest portions compared to the lower-ED meal with the smallest portions. The foods contributing the most to this increase were chicken, macaroni and cheese, and applesauce. The effects of meal portion size and ED on intake were not influenced by child age or body size, but were significantly affected by parental ratings of child eating behavior. Conclusion: Strategically moderating the portion size and ED of foods typically consumed by children could substantially reduce their energy intake without affecting acceptability. (author abstract)