Large food portions may be facilitating excess energy intake (EI) and adiposity among adults. The present study aimed to assess the extent to which El and amounts of foods consumed are influenced by the availability of different-sized food portions. A randomised within-subject cross-over, fully residential design was used, where forty-three (twenty-one men and twenty-two women) normal-weight and overweight adults were randomly allocated to two separate 4 d periods where they were presented with either `standard' or `large' food portions of the same foods and beverages. The main outcome measures were the amount of food (g) and El (MJ) consumed throughout each study period. Mean El over 4d was significantly higher on the large portion condition compared with the standard condition in the total group (59.1 (SD 6.6) v. 52.2 (so 143) MJ; P=0.020); men and women increased their El by 17% (10 (SD 6.5) MJ; P<0.001) and 10% (4 (SD 6.5) MJ; P=0.005) respectively when served the large food portions relative to the standard food portions. The increased intakes were sustained over the 4 d in the large portion condition with little evidence of down-regulation of El and food intake being made by subjects. Increased food portion size resulted in significant and sustained increases in El in men and women over 4 d under fully residential conditions. The availability and consumption of larger portions of food may be a significant factor contributing to excess El and adiposity.