Objective: Current guidelines for Kawasaki disease (KD) recommend intravenous immunoglobulin infusion and echocardiography. However, no previous studies have evaluated the relationship between the treating hospital’s concordance with guidelines and the prevention of coronary artery aneurysm in patients with KD. Study Design: KD patients between 2010 and 2013 were identified in a Japanese national inpatient database. Guideline concordance was defined as the proportions of patients who received echocardiography as well as treatment with both immunoglobulin and aspirin, which were divided into quartiles. Multivariable logistic regression analysis was conducted to examine the association between the guideline concordance and the occurrence of coronary artery aneurysm with adjustment for patient backgrounds. Results: In sum, 20 156 patients with KD were identified. The very high (>83.3%) concordance group had a lower rate of coronary artery aneurysm than the very low (<59.0%) concordance group (4.9% vs 9.9%; odds ratio, 0.45; 95% CI, 0.27 to 0.74; P = .002). Conclusion: All patients with KD should be treated according to the guidelines.