Two experiments were conducted to examine differences between people with Prader-Willi syndrome and an obese comparison group in choosing a larger quantity of food delivered after a delay (15, 30, or 60 seconds) or a small quantity of food delivered immediately. Individuals with Prader-Willi syndrome selected the larger food quantity. Choices by the obese comparison group tended to be equally divided between both options. Relative food preference influenced choosing the larger, delayed option in both groups. This effect appeared to be larger in the non-Prader-Willi syndrome group, but was not statistically significant. These results suggest perceived differences in food quantity may be an important determinant of food choice of people with Prader-Willi syndrome.