Background: UK policies aim to facilitate access to general psychiatric services for adults with intellectual disability (ID). If this is to be achieved, it is important to have a clear idea of the characteristics and proportion of people with ID who currently access specialist psychiatric services and the nature and extent of psychiatric disorders in this population. Methods: A cross-sectional study was carried out on all adults with ID using specialist services in Leicestershire and Rutland, UK, between 2001 and 2006. Characteristics of individuals seen by psychiatric services and the nature and prevalence of psychiatric disorders were investigated. Results: Of 2711 adults identified, 1244 (45.9%) accessed specialist psychiatric services at least once during the study period. Individuals attending psychiatric services were more likely to be older and to live in residential settings; they were less likely to be south Asian or to have mild/moderate ID. The prevalence of psychiatric disorders among the total study population was 33.8%; the most common disorders were behaviour disorder (19.8%) and autistic spectrum disorders (8.8%). Epilepsy was highly prevalent (60.8%) among those attending psychiatric services without a mental health diagnosis. Behaviour disorders and autistic spectrum disorders were more common in men and in adults with severe/profound ID, whereas schizophrenia and organic disorders were more common in women and in adults with mild/moderate ID. Depression was also more common in women with ID. Conclusions: Psychiatric disorders and specialist health problems are common among adults with ID and the profile of psychiatric disorders differs from that found in general psychiatry. Close collaboration between general and specialist service providers is needed if the current move towards use of general psychiatric services in this population is to be achieved. The measures should include a clear care pathway for people with ID and mental health problems to facilitate the smooth transfer of patients between specialist and generic mental health services and arrangements for joint working where input from both services is required. The commissioning framework for such processes should be in place with appropriate pooling of resources.