Background  In critical illness, serum total cortisol (TC) may not adequately reflect adrenal function because of reduced cortisol-binding globulin (CBG). Aim  To evaluate adrenal function of critically ill children, using free cortisol index (FCI), calculated free cortisol (cFC), and TC levels. Methods  Thirty-two critically ill and 36 healthy children were included. All children underwent the 1 μg cosyntropin test. TC and CBG levels were measured. Basal and peak TC, FCI, and cFC were determined. Results  Basal and peak TC, FCI, and cFC of critically ill children were significantly higher than those of the controls. Compared with TC, both basal and peak FCI and cFC of the patients were higher than those of controls to a greater degree. Use of FCI or cFC to diagnose adrenal insufficiency (AI) reduced the frequency of diagnosis of AI by 50%. Conclusion  FCI and cFC better reflect the dynamic changes of adrenal function of critically ill children.