The Behavioural Status Index (BEST-Index) has been introduced into Dutch forensic psychiatry to measure change in risk level of future violence. The BEST-Index is a structured observational measure that assesses aggressive behavior, degree of insight, social skills, self-care, and work and leisure skills during inpatient treatment. Thus far, limited information regarding the psychometric properties of the Dutch version of the BEST-Index is available. The present study examines the reliability and validity of the Dutch BEST-Index in a sample of 291 mentally disordered offenders admitted to a forensic psychiatric hospital. Interrater reliability was investigated in a sample of 182 raters. Findings show that the Dutch BEST-Index can be used reliably and is significantly associated with risk of future violence and institutional aggression. Furthermore, this study revealed a different and clearer factor structure compared with the original one. Further research is needed to examine how these derived factors predict future recidivism. (Contains 3 notes and 6 tables.)