Dutch and US teachers were questioned about their perceptions of the incidence and causes of ADHD, as well as their views of appropriate treatments. Dutch teachers were more likely than US teachers to think the etiology of the disorder lay in biochemistry, while US teachers were more likely to believe in a combination of environmental and biochemical factors. Both sets of teachers believed a combination of medical and behavioral interventions was the most effective treatment. Teachers were also asked to indicate the number of students in their classes who had been diagnosed with ADHD and to estimate the number of students who they thought might have ADHD. Both sets of teachers indicated that they thought there were students with undiagnosed ADHD in their classes, but Dutch teachers reported significantly fewer students in both the actual and perceived categories. Class size was related to US teacher perceptions, but not Dutch perceptions.