Personality disorder is prevalent among offenders with intellectual disabilities (ID), and it is associated with their risk for violence and recurrent offending behaviour. A new staff-rated instrument, the Personality Disorder Characteristics Checklist (PDCC), designed to screen for ICD-10 dissocial and emotionally unstable personality characteristics was evaluated for its reliability and validity, as applied to 129 male forensic patients with ID. Internal consistency and test–retest reliability were very good. Supportive evidence for concurrent and discriminant validity was obtained in conjunction with an established staff-rated instrument, but not for patient self-report measures. Construct validity support was found for the PDCC in association with violent offence and hospital assault history and in significantly accounting for the number of physical assaults in hospital, controlling for multiple covariates. Results for the new measure were favourable in comparison to established instruments. It clinical utility for treatment planning and for the management of risk is discussed.