Background: Anxiety disorders are known to be common in the general population. Previous studies with adults with intellectual disabilities (IDs) report a prevalence of general anxiety disorder ranging from less than 2% to 17.4%. Little is known about associated factors in this population. This study investigates point prevalence of anxiety disorders and determines the factors independently associated with them. Methods: Information was collected with 1023 adults with IDs who participated in a large-scale, population-based study. All had a comprehensive physical and mental health assessment. The point prevalence of anxiety disorders according to different diagnostic criteria was determined, as were independently associated factors by using logistic regression analysis. Results: Three point eight per cent (95% CI = 2.7-5.2%) of the cohort had an anxiety disorder at the time of assessment. Generalised anxiety disorder was the most common (1.7%), then agoraphobia (0.7%). Factors independently associated with having an anxiety disorder were not having any daytime employment, and having a recent history of life events. Having previously been a long-term hospital resident was independently associated with not having an anxiety disorder. Conclusions: Anxiety disorders are common in the ID population. At times of significant life events, it might be sensible for carers to consider proactively providing additional support and being vigilant to seek early health-care interventions should there be any suggestion of emerging mental ill-health. The study further highlights the range of mental ill-health that is experienced by the population with IDs, and therefore the need for appropriate care, supports and development of effective interventions.