Combination treatment with intravenous immunoglobulin (IVIG) plus prednisolone is effective for prevention of cardiovascular complications in children with Kawasaki disease (KD). However, administration of prednisolone for approximately 20 d in this regimen causes adrenocortical suppression in a high proportion of treated children. To establish a simple method to screen for this suppression, we performed a prospective study on 72 children with KD treated with this regimen in our institution from February 2012 to March 2014. By performing ROC analysis of 21 initial patients treated between February and June 2012, a serum cortisol value at 09:00 h of 5 mcg/dL was established as a threshold for intact adrenocortical function, which is equivalent to a peak serum cortisol value of higher than 15 mcg/dL in the CRH stimulation test. Then, we applied this screening test to 51 subsequent patients treated between July 2012 and March 2014. Approximately 90% of the patients with morning serum cortisol values above 5 mcg/dL 2 to 6 mo after the cessation of initial prednisolone treatment had peak serum cortisol values exceeding 15 mcg/dL, suggesting the efficacy of this approach.