Background: Achenbach & Rescorla (2003) recently developed the Adult Behavior Checklist (ABCL) to assess psychopathology in the general population. The ABCL should be completed by a proxy informant. The use of proxy informants, instead of self-reporting, makes the ABCL potentially suitable for the assessment of psychopathology in adults with intellectual disability. The aim of the present study was to examine reliability and validity of the ABCL in 124 adults with mild intellectual disability or low IQ, and severe challenging behaviour referred for residential treatment. Methods: The ABCL was completed by two independent informants to assess inter-rater reliability. To examine the validity of the ABCL, its relationship with three measures of functioning was assessed. Furthermore, association between scales of the ABCL and DSM-IV axis I disorders was examined. Results: The ABCL was reliable in terms of internal consistency of its scales, and inter-rater reliability. Relationships between clusters of axis I DSM-IV disorders and scales of the ABCL were found as expected. Moreover, ABCL scales predicted different measures of functioning. Conclusions: The ABCL appears to be a reliable and valid measure to assess psychopathology in persons with mild intellectual disabilities or low IQ, admitted for treatment in facilities for adults with mild intellectual disability and severe challenging behaviour.