This study investigates the prevalence and associated factors of mental ill-health among adults with intellectual disabilities. It found that 40.9% (clinical diagnoses), 35.2% (DC–LD), 16.6% (ICD–10–DCR) and 15.7% (DSM–IV–TR) of the 1023 participants had mental ill-health. The most common type was problem behaviours. Mental ill-health was associated with more life events, female gender, type of support, lower ability, more consultations, smoking, incontinence, not having severe physical disabilities, and not having immobility. It was not associated with deprived areas, no occupation, communication impairment, epilepsy, hearing impairment, or previous institutional residence. The study highlights that ICD–10–DCR and DSM–IV–TR undercount mental ill-health in this population compared with DC–LD. The study used a population-based approach with comprehensive assessments and detailed data collection. It found that mental ill-health is common among adults with intellectual disabilities, with over a third having mental ill-health. The study also identified factors independently associated with mental ill-health, such as being female, having profound or severe intellectual disabilities, experiencing more life events, having more consultations with healthcare professionals, and being a smoker. The study emphasizes the need for further research and the development of appropriate interventions, public health strategies, and policies to address the mental health needs of this population. The study also highlights the importance of considering the impact of developmental level on psychopathology in this population. The findings suggest that mental ill-health is a significant contributor to health inequalities among adults with intellectual disabilities. The study concludes that the most appropriate operationalised diagnostic criteria are those that most closely resemble the current 'gold standard' of clinical diagnosis by a specialist, provided comprehensive assessments and measurements have been used. The study also notes that the prevalence of mental ill-health is higher in this population compared to the general population. The study is limited by methodological factors, including the use of different diagnostic criteria and the cross-sectional design. The study recommends longitudinal cohort studies to determine predictive and aetiological factors for mental ill-health and to inform the development of interventions, public health strategy, and health and social care policy.This study investigates the prevalence and associated factors of mental ill-health among adults with intellectual disabilities. It found that 40.9% (clinical diagnoses), 35.2% (DC–LD), 16.6% (ICD–10–DCR) and 15.7% (DSM–IV–TR) of the 1023 participants had mental ill-health. The most common type was problem behaviours. Mental ill-health was associated with more life events, female gender, type of support, lower ability, more consultations, smoking, incontinence, not having severe physical disabilities, and not having immobility. It was not associated with deprived areas, no occupation, communication impairment, epilepsy, hearing impairment, or previous institutional residence. The study highlights that ICD–10–DCR and DSM–IV–TR undercount mental ill-health in this population compared with DC–LD. The study used a population-based approach with comprehensive assessments and detailed data collection. It found that mental ill-health is common among adults with intellectual disabilities, with over a third having mental ill-health. The study also identified factors independently associated with mental ill-health, such as being female, having profound or severe intellectual disabilities, experiencing more life events, having more consultations with healthcare professionals, and being a smoker. The study emphasizes the need for further research and the development of appropriate interventions, public health strategies, and policies to address the mental health needs of this population. The study also highlights the importance of considering the impact of developmental level on psychopathology in this population. The findings suggest that mental ill-health is a significant contributor to health inequalities among adults with intellectual disabilities. The study concludes that the most appropriate operationalised diagnostic criteria are those that most closely resemble the current 'gold standard' of clinical diagnosis by a specialist, provided comprehensive assessments and measurements have been used. The study also notes that the prevalence of mental ill-health is higher in this population compared to the general population. The study is limited by methodological factors, including the use of different diagnostic criteria and the cross-sectional design. The study recommends longitudinal cohort studies to determine predictive and aetiological factors for mental ill-health and to inform the development of interventions, public health strategy, and health and social care policy.