Objective Examine the effect of weekday exposure over 6 months to different lunch sizes on energy intake and body weight in a free-living sample of working adults. Methods Adults (n=233) were randomly assigned to one of three lunch size groups (400 kcal, 800 kcal, and 1,600 kcal) or to a no-free lunch control group for 6 months. Weight and energy intake were measured at baseline, and months 1, 3, and 6. Results Lunch energy was significantly higher in the 800 and 1,600 kcal groups compared with the 400 kcal group (P<0.0001). Total energy was significantly higher for the 1,600 kcal group compared with the 400 and 800 kcal groups (P=0.02). Body weight change at 6 months did not significantly differ at the 5% level by experimental group (1,600 kcal group: +1.1 kg (SD=0.44); 800 kcal group: -0.1 kg (SD=0.42); 400 kcal group: -0.1 kg (SD=0.43); control group: 1.1 (SD=0.42); P=0.07). Weight gain over time was significant in the 1,600 kcal box lunch group (P<0.05). Conclusions Weekday exposure for 6 months to a 1,600 kcal lunch caused significant increases in total energy intake and weight gain.