Purpose: Mental ill health in adults with intellectual disability (ID) is a neglected field in psychiatry and thus still widely understudied. This paper provides data on the prevalence of mental illness and problem behaviour and analyses support needs, mental health service use and psychotropic medication in a representative sample of adults with mild to moderate ID. Methods: A set of well-established instruments was used to assess the main parameters in n = 371 participants recruited within a cross-sectional epidemiological multicentre study using a stratified randomised cluster sampling. Results: Point prevalence of mental disorders was 10.8 %, that of problem behaviour 45.3 %. Most study participants needed help in specific lower order need areas (e.g., money budgeting, food, accommodation), and these need areas were mostly rated as met. The highest ratios of unmet to met need were found with respect to sexuality issues and with respect to mental health problems. The focus of psychiatric treatment was psychotropic medication. Conclusions: Referring to ICD-10 based diagnostic criteria and consequently avoiding confusing problem behaviour with mental disorders, point prevalence of mental disorders was lower than in the general population. A systematic deficit in meeting mental health problems in adults with ID indicates the need for implementing strategies to maximise the quality of identification and management of mental disorders.