Background: Little is known about client characteristics that are related to outcome during inpatient treatment of adults with mild intellectual disability (ID) and severe behavioural problems. Method: We explored variables that were related to a change in behavioural problems in 87 individuals with mild ID during inpatient treatment in facilities for adults with mild ID and severe behavioural problems. Emotional and behavioural problems were measured using the Adult Behaviour Checklist (ABCL) and relationships between a change in ABCL total scores and other variables (e.g. age, adaptive skills, and presence of personality disorder) were explored. Results: For the sample as a whole, we found a decrease in emotional and behavioural problems within a 1-year period of inpatient treatment (following a 3-month observation and diagnostic period). Male clients, clients without a personality disorder classification, and/or clients who showed more improvement in adaptive and social functioning, showed a larger decrease in emotional and behavioural problems than other clients. Gender and personality disorder classification appeared to be the most important predictive factors. Conclusion: Clients with mild ID and severe behavioural problems may benefit from inpatient treatment. Those with a personality disorder and/or female clients may benefit less from such a program. Results of such studies may have consequences for service provision and management. Limitations of this study were the lack of data on the quantity and quality of the treatment package and unknown reliability and validity of Axis-I and personality disorder diagnoses.