OBJECTIVE: This study investigated the effect on energy intake of increasing the portion size of a food served as a discrete unit. DESIGN: A within-subject design with repeated measures was used. SUBJECTS/SETTING: The sample comprised 75 young adults (37 females and 38 males) from a university community. INTERVENTION: Individuals ate lunch in the lab once a week for 4 weeks. Each week, they were served one of four sizes of a deli-style sandwich (6, 8, 10, or 12 inches), of which they could eat as much as they wanted. MAIN OUTCOME MEASURES: Energy intakes were determined for each meal, as were ratings of hunger and satiety before and after each meal. STATISTICAL ANALYSES PERFORMED: A linear mixed model with repeated measures was used. The influence of subject characteristics was examined using analysis of covariance. RESULTS: The portion size of the sandwich significantly influenced lunch intake for both males and females (P<.0001). The majority of individuals consumed the entire 6-inch sandwich. When served the 12-inch sandwich, compared with the 8-inch sandwich, females consumed 12% more energy (74 kcal) and males consumed 23% more energy (186 kcal). Despite these differences, ratings of hunger and fullness were not significantly different after eating the 12-inch and 8-inch sandwiches. APPLICATIONS/CONCLUSIONS: These results suggest that increasing the portion size of a food served as a discrete unit leads to increased energy intake at a single meal without differentially influencing ratings of hunger and satiety. Dietitians should educate their clients about strategies to moderate the effect on intake of increased portions of high-calorie foods.