Attitudes play a pivotal role in the inclusion of children with Attention Deficit/Hyperactivity Disorder (ADHD) in mainstream schools but little is known about factors that influence these. This study investigated the effect of ADHD knowledge and stigma on professionals attitudes towards mainstream inclusion. Teachers, support staff, school managers and educational psychologists completed questionnaires assessing ADHD knowledge, stigma and attitudes towards inclusion. Psychologists displayed more knowledge, had less stigmatising beliefs and more inclusive attitudes than other professions. Regression analyses revealed those with more knowledge of ADHD and less stigma held more positive attitudes towards mainstream inclusion. Results have implications for how to promote inclusive beliefs about ADHD.