Since the middle of the 20th century, there have been several heterogeneous studies of recidivism by offenders with intellectual disabilities (ID) who have been in specialist mental health services after an index offence. Although some were long term, as befits a chronically needy group, laws and services have changed in that time. It may no longer be appropriate to rely on findings from the 1960s and 1970s. To compare mental health presentations and recidivism between male sex offenders, men convicted of other offences and female offenders from a 1986–2008 cohort of offenders referred to specialist forensic ID services in Scotland. A 20-year follow-up of an assessment and treatment service for 309 offenders with ID (156 sex offenders, 126 non sexual male offenders and 27 women) was conducted. Sex offenders were more likely to be referred from the courts and had lower anger levels. Women were more likely to be diagnosed with mental illness. There was a split between groups with sex offenders committing few nonsexual offences and the other groups showing few sexual offences. During the study period, 16% of sexual offenders, 43% of male nonsexual offenders and 23% of women committed at least one further offence. Following assessment and treatment there was a 90–95% reduction in offending incidents. Although recidivism rates are disappointing, harm reduction data suggests that assessment and treatment for offenders with ID can be highly successful in terms of public safety.