BackgroundPersons diagnosed with attention-deficit hyperactivity disorder (ADHD) have been found to have an increased risk of suicidal behaviour, but the pathway remains to be thoroughly explored.AimsTo determine whether persons with ADHD are more likely to present with suicidal behaviour (i.e. suicide attempts and deaths by suicide) if they have a comorbid psychiatric disorder.MethodUsing nationwide registers covering the entire population of Denmark, this cohort study of 2.9 million individuals followed from 1 January 1995 until 31 December 2014, covers more than 46 million person-years. All persons aged ≥10 years with Danish-born parents were identified and persons with a diagnosis of ADHD were compared with persons without. Incidence rate ratios (IRRs) were calculated by Poisson regression, with adjustments for sociodemographics and parental suicidal behaviour.ResultsPersons with ADHD were followed for 164 113 person-years and 697 suicidal outcomes were observed. This group was found to have an IRR of suicidal behaviour of 4.7 (95% CI, 4.3–5.1) compared with those without ADHD. Persons with ADHD only had a 4.1-fold higher rate (95% CI, 3.5–4.7) when compared with those without any psychiatric diagnoses. For persons with ADHD and comorbid disorders the IRR was higher yet (IRR: 10.4; 95% CI, 9.5–11.4).ConclusionsThis study underlines the link between ADHD and an elevated rate of suicidal behaviour, which is significantly elevated by comorbid psychiatric disorders. In sum, these results suggest that persons with ADHD and comorbid psychiatric disorders are targets for suicide preventive interventions.Declaration of interestNone.