Introduction: There is reason to believe that many individuals with substance use disorders (SUD), who are in contact with services, have an undiagnosed intellectual disability. The aim of the present study was to investigate the prevalence of intellectual and borderline intellectual disabilities (ID/BID) among inpatients with SUD and to further explore their characteristics. Methods: Ninety-one SUD inpatients aged 19-64 participated in the study. SUD was diagnosed according to the "ICD"-10. ID/BID was diagnosed through the Wechsler Adult Intelligence Scale, the Vineland Adaptive Behavior Scale, and self-reported childhood learning difficulties. Results: ID/BID prevalence was up to 30%. There were significant differences between ID/BID and non-ID groups for several social variables. With the exception of substance-use relapse, there were no significant differences in substance-related factors. Conclusions: ID/BID prevalence in SUD might be high. These conditions often go undiagnosed in SUD treatment. The conditions and associated difficulties should be identified for better understanding and individually adapted treatment.