Introduction: A biomarker predicting further development risk of delirium would be of interest. Cortisol, as a lipid base blood hormone playing role in hypothalamo-pituitary-adrenal axis, might be a good indicator. However, relationship between baseline blood cortisol level and delirium formation has not been yet investigated in ICU. Methods: Patients >18 years followed >48 hours in Hacettepe University Anesthesiology ICU from January 2017 to 2020 were included. Delirium was diagnosed according to CAM-ICU scale (D[+] vs D[-]). Primary outcome was whether baseline (at admission) blood cortisol level differs between groups and secondary one was to detect predictive factors associated with delirium. Results: 125 out of 562 patients were included in which 38 (30%) were diagnosed with delirium. Mean age was 52±25 years in D [+] group [Female (F): 17 (45%)] while it was 64±21 years in D [-] group [F: 39 (45%)] (p=0.11 and p=0.57 for age and gender, respectively). Groups did not differ regarding laboratory parameters including baseline serum cortisol level, LOS, and mortality. Only rates of previous alcohol use and steroid administered in ICU, unlike pneumonia as a cause of admission, were higher in D [+] groups while compared to D [-]. However, no parameter has persisted as an independent factor for predicting delirium while adjusted for age (>50 years), gender, and blood cortisol level in binary backward logistic regression analysis. Discussion: It seems baseline blood cortisol level may not be a good predictor regarding further delirium formation in mixed medical and surgical ICU patients.