Background: Many offenders with intellectual disabilities have substance use issues. Offending behaviour may be associated with substance use. Materials and Methods: Prisoners with and without intellectual disabilities were compared in terms of their substance use prior to imprisonment, the influence of substance use on offending, and their participation in alcohol and drug treatment programmes. Results: Substance use was similar in prisoners with and without intellectual disabilities in the year prior to their current prison terms. Prisoners with intellectual disabilities were much less likely to report that substance use was an antecedent to the offences leading to their imprisonment. The completion rate of alcohol and drug treatment programmes was much lower for those with intellectual disabilities. Conclusions: Substance use may be as common in prisoners with intellectual disabilities as those without this condition. Services may need to reflect on whether their treatment programmes are meeting the needs of all prisoners.