The purpose of this study was to determine the amount of physical and sedentary activity normal-weight and at-risk-for/overweight boys perform when alone, with a peer of similar weight and with a peer of different weight. Participants included boys, ages 8-12 years, classified as either normal-weight (<85th BMI percentile; N = 12) or at-risk-for/overweight (<85th BMI percentile; N = 12). At-risk-for/ overweight boys allocated a greater amount of time to sedentary activities and accumulated fewer accelerometer counts than normal-weight boys in the alone condition. Once paired with a peer of either similar or different weight there were no differences between groups. These results indicate the presence of an unknown peer has a positive effect on at-risk-for/overweight children's physical activity behavior.