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Background: Although comorbid medical diseases are important determinants of outcome among the critically ill, the role of psychiatric comorbidity is not well defined. The objective of this study was to determine the occurrence of psychiatric comorbidity and its effect on the outcome of patients admitted to adult intensive care units (ICU) in Queensland. Methods: Admissions among adults to 12 ICUs in Queensland during 2015–2021 were included and clinical and outcome information was obtained through linkages between the ANZICS Adult Patient Database, the state-wide Queensland Hospital Admitted Patient Data Collection, and death registry. Results: A total of 89,123 admissions were included among 74,513 individuals. Overall, 7,178 (8.1%) admissions had psychiatric co-morbidity with 6,270 (7.0%) having one major psychiatric diagnosis and 908 (1%) having two or more. Individual diagnoses of mood, psychotic, anxiety, or affective disorders were present in 1,801 (2.0%), 874 (1.0%), 3,241 (3.6%) and 354 (0.4%) admissions respectively. Significant differences were observed among the main groups (mood, affective, anxiety, psychotic, or multiple disorders) and those without psychiatric comorbidity with respect to main diagnosis, Acute Physiology and Chronic Health Evaluation (APACHE II) score, sex, age, and medical comorbidity. Crude 30-day case-fatality rates were significantly lower (5.1%) compared to the general ICU population (10.1%) (p < 0.001). After controlling for confounding variables in the logistic regression model, patients with psychiatric comorbidity were at lower odds of death. Conclusions: Psychiatric comorbidity is common among ICU presentations and is associated with a lower risk of death. This association is likely to be more complex than being a simple protective factor, and future research needs to further delineate how psychiatric comorbidity informs outcomes of specific ICU presentations. [ABSTRACT FROM AUTHOR]