Diagnosing Attention Deficit Hyperactivity Disorder (ADHD) in people with intellectual disability (ID) remains challenging. The Diagnostic and Statistical Manual of Mental Disorder V (DSM V) classification system is often used to diagnose ADHD in the general population; however, the presence of ID and other associated conditions such as autism and communication difficulties can make it difficult to apply the DSM V criteria in people with ID. Therefore, diagnosing ADHD in people with ID is often made using clinical judgement and/or the application of diagnostic criteria. There are no studies comparing the diagnostic accuracy of clinical judgement and the use of DSM V criteria in people with ID and ADHD. The aims of the study were to compare the accuracy of the diagnosis of ADHD in people with ID according to the DSM V criteria versus clinical judgement and to determine which criteria are more reliable. A questionnaire was developed using five fictional case scenarios of people with ID. Questionnaires were presented to practising psychiatrists chosen as a convenience sample in the United Kingdom over a period of 12 months. Case scenarios were developed and agreed to be positive or negative for ADHD by the study authors prior to rating by clinicians. The clinicians were asked to read the scenarios and to make a judgement on the cases regarding the symptoms of ADHD. They were then presented with the 18 DSM V criteria of ADHD and asked to select the criteria they considered were present in each scenario. Sensitivity, specificity, likelihood ratios and predictive values for both the DSM V criteria and clinical opinions were calculated for correctly identifying the exemplar cases. The data showed strong sensitivity [0.82 95% confidence interval (CI) 0.74-0.89] and high specificity (1.00 95% CI 0.95-1.00) for the raters' clinical opinion. In contrast, the DSM V criteria alone, as assessed by the raters, did not reliably provide ADHD diagnoses, with a sensitivity of only 0.23 (95% CI 0.15-0.31). This difference in sensitivity between the two was statistically significant at P < 0.001. The study results suggest that clinical opinion is the 'gold standard' at present in diagnosing ADHD in adults with ID in the absence of a validated diagnostic tool in this group. Further studies are needed to understand how symptoms of ADHD can be presented differently in people with ID. DSM V criteria for ADHD may need to be adapted according to the severity of ID and other neurodevelopmental disorders. © 2019 MENCAP and International Association of the Scientific Study of Intellectual and Developmental Disabilities and John Wiley & Sons Ltd.