Context: Fast food consumption has increased greatly among children in recent years, in tandem with the obesity epidemic. Fast food tends to promote a positive energy balance and, for this reason, may result in weight gain. However, if fast food and obesity are causally related, the question arises of why some children who frequently eat fast food do not become overweight. Objective: To test the hypothesis that overweight adolescents are more susceptible to the adverse effects of fast food than lean adolescents. Design and Setting: In study 1, we fed participants an "extra large" fast food meal in a naturalistic setting (a food court). The participants were instructed to eat as much or little as desired during this 1-hour meal. In study 2, we assessed energy intake under free-living conditions for 2 days when fast food was consumed and 2 days when it was not consumed. Data were collected in Boston, Mass, between July 2002 and March 2003. Participants: Overweight (n=26) and lean (n=28) adolescents aged 13 to 17 years. Overweight was defined as a body mass index exceeding sex- and age-specific 85th percentiles based on the 2000 Centers for Disease Control and Prevention growth charts. Main Outcome Measures: Energy intake determined by direct observation in study 1 and by unannounced 24-hour dietary recalls, administered by telephone, in study 2. Results: In study 1, mean (SEM) energy intake from the fast food meal among all participants was extremely large (1652  kcal), accounting for 61.6% (2.2%) of estimated daily energy requirements. Overweight participants ate more than lean participants whether energy was expressed in absolute terms (1860  vs 1458  kcal, P=.02) or relative to estimated daily energy requirements (66.5% [3.1%] vs 57.0% [2.9%], P=.03). In study 2, overweight participants consumed significantly more total energy on fast food days than non-fast food days (2703  vs 2295  kcal/d; +409  kcal/d; P=.02), an effect that was not observed among lean participants (2575  vs 2622  kcal/d; -47  kcal/d; P=.76). Conclusions: In this study, adolescents overconsumed fast food regardless of body weight, although this phenomenon was especially pronounced in overweight participants. Moreover, overweight adolescents were less likely to compensate for the energy in fast food, by adjusting energy intake throughout the day, than their lean counterparts.