Background Delirium is a common and deleterious complication in critically ill patients after surgery. The purpose of this study was to determine the incidence and risk factors of delirium in critically ill patients after non-cardiac surgery, and to investigate the relationship between the serum cortisol level and the occurrence of postoperative delirium. Methods In a prospective cohort study, 164 consecutive patients who were admitted to the surgical intensive care unit after non-cardiac surgery were enrolled. Baseline characteristics and perioperative variables were collected. Blood samples were obtained on the first postoperative day and serum cortisol concentrations were measured. Delirium was assessed using the Nursing Delirium Screening Scale until the seventh postoperative day or the disappearance of delirious symptoms.Results Postoperative delirium occurred in 44.5% of patients (73 of 164). The median time to first onset of delirium is 0 (range 0 to 5 days) and the median duration of delirium is 3 (1 to 13) days. Independent risk factors of postoperative delirium included increasing age (odds ratio (OR) 2.646, 95% confidence interval (CI) 1.431 to 4.890, P=0.002), a history of previous stroke (OR 4.499, 95%CI 1.228 to 16.481, P=0.023), high Acute Physiology and Chronic Health Evaluation Ⅱ score on surgical intensive care unite admission (OR 1.391, 95%CI 1.201 to 1.612, P<0.001), and high serum cortisol level on the 1 st postoperative day (OR 3.381, 95%CI 1.690 to 6.765, P=0.001). The development of delirium was linked to higher incidence of postoperative complications (28.8% vs. 7.7%, P<0.001), and longer duration of hospitalization (18 (7 to 74) days vs. 13 (3 to 48) days, P <0.001).Conclusions Delirium was a frequent complication in critically ill patients after non-cardiac surgery. High serum cortisol level was associated with increased incidence of postoperative delirium.