Background The relationship between inflammation and delirium remains to be determined.The purposes of this study were to investigate the association between serum interleukin-6 levels and the occurrence of delirium in elderly patients after major noncardiac surgery.Methods A total of 338 elderly patients (60 years of age and over) undergoing major noncardiac surgery were enrolled.Blood samples were obtained before anesthesia and in the first postoperative ming and serum interleukin-6 concentrations were measured.Delirium was assessed twice daily by the confusion assessment method for the Intensive Care Unit during the first three postoperative days.Survival analyses were performed to assess the relationship between the serum IL-6 level and the occurrence of postoperative delirium.Results Postoperative delirium occurred in 14.8％ (50 of 338) of patients.High serum interleukin-6 levelsafter surgery were significantly associated with increased risk of the occurrence of postoperative delirium (hazard ratio 1.514,95％ confidence interval 1.155-1.985,P=0.003).Other independent predictors of delirium included increasing age,poor preoperative New York Heart Association classification,low preoperative Mini-Mental State Examination score,and high total postoperative Visual Analogue Scale pain score.Patients who developed delirium had a prolonged hospital stay after surgery.Conclusions Delirium is a frequent complication in elderly patients after noncardiac surgery.High serum interleukin-6 level after surgery is associated with increased risk of the occurrence of postoperative delirium.