As recently as the 1980s, it was believed that people with intellectual disability (ID) did not have the cognitive capacity to experience mental health problems and that behavioural disturbances were attributable to ID. Thankfully, the literature caught up with clinical experience and it is now accepted that people with such disability do experience the same mental ill-health as those without it and that they are probably more vulnerable. Epidemiological studies estimating the coexistence of attention deficit hyperactivity disorder (ADHD) in this population were mired with difficulties as they required valid and reliable measurements of both the learning disability and ADHD, which were not always available. As a result, rather unhelpfully, the rate of coexistence was initially calculated to be 10–92%.