As portion size (PS) increases, so does food intake. The effect of decreasing PS on food intake in a nonlaboratory setting is unknown. This 5-week study sought to determine whether decreasing PS resulted in decreased intake of the same food, and if so, at what point further PS reductions might lack benefit. It also assessed effects of PS reduction on food production and waste in a university all-you-can-eat dining facility (DF). Subjects were primarily freshmen who regularly ate lunch at the DF, and self-selected French fries (FF) presented in individual paper bags, portioned originally at 88 g, and decreased approximately 15 g/week for 3 weeks. Diners were covertly observed choosing one or more bags. Total FF production and plate waste (PW) were determined daily. Decreasing PS resulted in significant decreases in consumption per diner (P < 0.05) and PW (P < 0.05), and nonsignificant decreases in total FF consumption and production. PS was positively correlated with consumption per diner (r = 0.897, P = 0.001) and PW (r = 0.852, P = 0.001), but inversely correlated with number of diners choosing >or=2 bags (r = -0.809, P = 0.003). Total FF production was positively correlated with PW (r = 0.728, P = 0.011). This study shows that reducing PS of a particular item in an all-you-can-eat environment results in reduced intake of that food for most individuals, and that reducing PS reduces PW and food production.