Objective: Studies have identified an exacerbation of ADHD deficits under specific laboratory conditions. Less is known about the significance of such contextual factors in relation to everyday functioning in naturalistic settings. Method: This study investigated the differential impact of classroom "idle time"—periods when students are not actively engaged or waiting for a task—on the behavior of 31 children with ADHD (25 boys and 6 girls; aged 6-12 years) and 31 sex- and age-matched typically developing classmates, who were simultaneously observed in their normal classroom during two school days. Results: Both groups experienced the same amount of idle time (12% of the time). During idle time, however, levels of hyperactivity and noisiness increased significantly more in children with ADHD than in their classmates (p < .05). Conclusion: Findings highlight the differential susceptibility of ADHD children to classroom idle time. Classroom interventions might consider targeting specifically these periods to reduce disruptive behavior in these children. (J. of Att. Dis. 2013; XX(X) 1-XX)