For nurses working in mental health service settings, it is a priority to perform patient assessments to identify patients' general and behavioural risks and nursing care complexity using objective criteria, to meet the demand for care and to improve the quality of service by reducing health threat conditions to the patients' selves or to others (adverse events). This study highlights that there is a relationship between the complexity of psychiatric patient care, which was assigned a numerical value after the nursing assessment, and the occurrence of psychiatric adverse events in the recent histories of the patients. The results suggest that nursing supervision should be enhanced for patients with high care complexity scores.