Children with Attention-Deficit/Hyperactivity Disorder (ADHD) evidence many problems in the classroom, including difficulty staying seated and trouble with organizational tasks. Such behaviors cause impairment for the child in their academic functioning and place a burden upon their teachers. Despite the large evidence base for classroom behavioral interventions, teachers often lack specific training on and accurate knowledge about ADHD. Teacher in-service training is routinely utilized to inform school professionals about a number of special topics. However, the efficacy of such training for ADHD has not been established. The present study examined the efficacy of brief in-service training in improving teacher knowledge about ADHD, use of behavior management techniques, and levels of stress related to teaching a child with ADHD. Six schools in the Washington, D.C. metropolitan area participated. Teachers at these schools were randomly assigned to receive in-service training immediately or to a waitlist control group that received in-service training one month later. Teacher ratings of ADHD knowledge, use of behavior modification techniques, and stress were measured pre in-service intervention and 1 month post in-service intervention. Behavioral observations of behavior modification strategies were gathered on a random subset of teachers from each school at each time point. Mixed model analyses of variance were used to examine the effects of the intervention on ADHD knowledge, use of behavior modification techniques, and teacher stress. A Treatment Group X Time interaction was found for teacher-reported ADHD knowledge, such that the immediate in-service group reported significantly increased knowledge from pre to post in-service intervention while the waitlist control group did not. Teacher use of reported behavior modification techniques appeared to change for special education teachers only. Stress did not change as a result of the intervention. Observational data did not correlate highly with teacher self-report data. Limitations of this study include the use of a newly-developed measure of ADHD knowledge that requires psychometric testing and the lack of observations of child behavior. Future studies should examine ways to better measure and promote actual behavior change among teachers of children with ADHD.