Following the closure of the large mental handicap hospitals in the UK, the majority of people with intellectual disability (ID) are currently living in the community. However, people with ID who also exhibit challenging behaviour (CB) have been the most difficult-to-place group and use a large amount of service resources. A variety of service options have been proposed for the assessment and treatment of CBs, but there is little information on the effectiveness of these alternatives. The Mental Impairment Evaluation and Treatment Service (MIETS) is one of these service options and the aim of the present study is to describe and evaluate this service. The present authors studied the first 64 patients admitted to MIETS following its opening. A within-subject comparison research design was used. Demographic and clinical data were obtained from case records and the effectiveness of MIETS interventions was evaluated by comparing the number of incidents of challenging behaviour, the use of seclusion, and the place of residence before and after the MIETS intervention. Only 10 (17.5%) of the patients had been admitted from community facilities, but 48 (84.2%) of the patients were discharged to community placements (P < 0.0001). The MIETS also significantly reduced the frequency and severity of challenging behaviours (P < 0.0001). It is concluded that the MIETS is an effective treatment model for people with ID and CB, and that there is no place for therapeutic nihilism in this difficult-to-place group of patients.