BACKGROUND AND OBJECTIVES: Kawasaki disease (KD) is the leading cause of acquired heart disease, which predominantly occurs in children under the age of 5 years. However, there are fewer cases of KD in infants younger than 6 months, which makes it difficult to diagnose. We investigated the clinical manifestations of KD in this age group and compared them with those in the older age group. SUBJECTS AND METHODS: We retrospectively reviewed the medical records of 239 patients with KD admitted to the Hallym Sacred Heart Hospital from January 2013 to June 2015. The data were categorized into 2 groups by age: Group A (≤6 months, n=26) and Group B (>6 months, n=213). RESULTS: Group A had a longer hospitalization period than Group B (6.69 vs. 5.19 days, p=0.002). Group A had fewer clinical manifestations upon admission, due to which there was a higher incidence of incomplete KD (1.88 vs. 3.54 of the diagnostic criteria, p<0.05; and 19.2% vs. 4.2% incomplete KD incidence, p=0.002). The rate of cardiac complications was higher in Group A (30.8% vs. 11.7%, p=0.011). CONCLUSION: The rate of cardiac complications of the patients younger than 6 months was significantly higher than that of the older patients. Therefore, infants younger than 6 months with unexplained fever for more than 5 days should be suspected as having KD, even if the principal clinical features are not fully presented. Echocardiography must be appropriately used for diagnosis of KD in suspected patients.