In this study, psychometric properties of the Health of the Nation Outcome scales (HoNOS) and Health of the Nation Outcome Scales for People with Learning Disabilities (HoNOS-LD) were investigated in a sample (n = 79) of (young) adults with mild to borderline intellectual disability (ID) and severe behavior and mental health problems who were admitted for treatment. Informant pairs were 14 direct care staff and 15 psychiatrists/psychologists who completed both the HoNOS and HoNOS-LD as well as other scales (i.e., Social Functioning Scale for the Mentally Retarded [SRZ-P], Adult Behavior Checklist [ABCL]). Generally, internal consistency was fair to good for both scales, and intraclass coefficients for (sub)scales ranged from fair to good. Low Intraclass Correlation Coefficients (ICCs) were found for several items from both scales. Outcomes pertaining to criterion validity were mixed. Finally, although we found a positive relationship between scores on the HoNOS and HoNOS-LD and those of the ABCL, the relationships between scores of both HoNOS and HoNOS and the SRZ-P were negative. It is concluded that both HoNOS and HoNOS-LD may be used in clients with mild to borderline ID and that outcomes may depend on the type of rater. The HoNOS-LD may be more suitable for assessing general functioning of clients with mild to borderline ID than the HoNOS. (Contains 5 tables.)