Behavior modification is an evidence-based treatment for children with attention-deficit/hyperactivity disorder (ADHD). Time-out from positive reinforcement is one behavior-modification procedure commonly recommended to manage disruptive or noncompliant behavior. This investigation examined the effects of time-out on children's behavior within the context of an intensive summer treatment program at two sites. Short (5-minute), long (15-minute), and escalating/de-escalating time-out conditions were compared to no time-out for 71 children. Results indicated that the time-out conditions were superior to no time-out in reducing the occurrence of intentional aggression, intentional destruction of property, and repeated noncompliance in the recreational and classroom settings regardless of child age or parent reported oppositional or conduct problems. There were no effects of time-out on staff member ratings of time-out effectiveness or the pleasantness of staff-child interactions. All three time-out conditions were equally effective in the group analyses, though analyses of normalization of functioning indicated there were individual differences in response to the time-out conditions. The results indicated that time-out is an effective means of reducing negative behavior in children with ADHD.