Background and purpose: The purpose of this study was to assess the level of mental disorders and challenging behaviour in individuals with intellectual disability (ID) supported by specialized services, but without a prior psychiatric diagnosis, and to compare the levels of different disorders depending on the severity of ID. Methods: This is a cross-sectional study (N = 142) of population with ID. Inclusion criteria were the following: adult patients with ID and with no previous psychiatric diagnosis prior to this survey. The Wechsler Adults Intelligence Scale-II, the Psychiatric Assessment Schedule for Adults with Developmental Disability checklist and clinical interview, the Diagnostic Assessment for the Severely Handicapped scale and the Inventory for Client and Agency Planning were the assessment tools. Results: A previously undiagnosed mental disorder was found in 29.6% of the sample. The most prevalent mental disorders were major depressive and anxiety disorders. An association between psychiatric comorbidity and challenging behaviour was found only for mild/moderate ID, especially for affective disorders. Conclusions: The presence of a psychiatric as well as a medical comorbidity is associated with severe ID, unlike challenging behaviour. Clinical limitations of the study have been discussed.