Intellectually disabled offenders (IDO) are a poorly served and under-recognized group, who are likely to require long-term specialist treatments and interventions. Method This prospective study investigated the characteristics and factors that influence outcome in this group, with particular reference to therapeutic interventions. Sixty-one individuals were identified from contact with either (1) specialist health and social services for people with intellectual disability (ID) or (2) nonspecialist services in the criminal justice or (forensic) mental health/social service systems. The participants were assessed at baseline and after a mean of 10 months in order to compare recidivism rates and the impact of therapeutic interventions. The findings suggest that IDO start offending at an early age, that they frequently have a history of multiple offences, and that sex offending and arson are over-represented offence types. Those participants recruited from nonspecialist ID services had significantly higher IQs and were less likely to have had contact with community social and health support agencies. Despite the high rates of psychopathology, there was little evidence for efficacy of therapeutic interventions, which, where offered, appeared to be of a nonspecific nature. At second interview, approximately half of the sample had re-offended.