This study examined group differences of 49 boys ages 6 to 11 years with and without attention deficit hyperactivity disorder (ADHD) in emotion regulation during frustrating peer competition. Half of all boys in each group were explicitly instructed to hide their feelings if they became upset during the competition. Behavioral inhibition, both before and after the competitive task, was examined using the Stop Signal Task (SST), and emotion regulation was assessed via structured observation data. Effect sizes indicated that impulsive ADHD boys displayed greater disinhibition and were less effective at emotion regulation than comparison boys. In addition, boys with ADHD were unsuccessful in masking their emotions even when instructed to do so. In contrast, comparison boys were more successful at emotion regulation when given instruction to self-regulate, and these regulatory attempts predicted later inhibitory control. Findings are discussed in the context of current ADHD-related theories of inhibitory deficit, and suggestions for future research are provided.