Background: Evaluative studies have shown that special units for people with intellectual disabilities (ID) who have challenging behaviour have advantages and disadvantages. There has been no survey of their number or characteristics for nearly 20 years. Methods: A questionnaire was sent to all National Health Service trusts that had ID inpatient beds, and all private or voluntary healthcare establishments providing services for people with mental health problems or ID. This asked for information about the unit, its residents and the views of the unit manager. Results: Forty-four agencies confirmed that they provided assessment and treatment units, of which 38 returned questionnaires. These units served 333 people, of whom 75% had mild or moderate ID. A quarter had been there for more than 2 years. Forty per cent of residents had a discharge plan, and 20% had this and the type of placement considered ideal for them in their home area. The main strengths of the units were identified as the knowledge and experience of the staff and having sufficient staff; the main problems as inappropriate admissions, bed-blocking and the relationship with other services; difficulties with recruiting and retaining staff; the location and environment of the unit; and the mix of residents. Conclusions: There has been an increasing rate of provision of special units, which now predominantly serve people with moderate or mild ID. This model of service provision is becoming more widespread, but the potential problems identified 20 years ago are still present. Areas are identified for further research.