When the Adapted Barratt-Stanford Records Review for Impulsive Premediated Aggression Assessment was used to rate aggression in mentally ill patients who required maximum-security hospitalization, moderately good inter-rater reliability was achieved. The study results sufficiently confirmed the potential use of this instrument in providing a more systematic and objective assessment of aggression in patients with mental illness compared with clinical assessment alone. The results indicated that when impulsive and premeditated aggression was distinguished in such patients, the vast majority exhibited predominantly impulsive aggression. Of the minority with predominantly premeditated aggression, 97.4 percent suffered from a thought disorder. Of the 67 patients with a thought disorder, nearly 1 in 5 had predominantly premeditated aggression, a much higher ratio than in all other disorders combined. This suggests that suspicious, paranoid, or otherwise delusional thinking contributed to the thought-disordered patients' potential to assault with premeditation. However, the packets for 65 percent of transferred patients to Chester Mental Health Center lacked documents with sufficiently detailed description of the violent act for the instrument to be applied. This could have created selection bias. The study subjects were 250 patients with successive admissions to the Chester Mental Health Center, the maximum security hospital for Illinois. These patients were admitted during a 5-year period. The adapted Barratt-Stanford Records Review for impulsive/predmeditated aggression assessment was used to collect data from all written documents sent prior to, or in conjunction with the transfer, the transfer packet, the medical record, and any other records or documents included. 26 references and appended Adapted Barratt-Stanford Records Review for Impulsive/Premeditated Aggression Assessments