Kawasaki disease (KD) patients younger than 1 year of age are at especially high risk of developing coronary artery abnormalities (CAA). To define the clinical characteristics of this group, as well as the risk factors predisposing them to CAA, we reviewed the medical records of 136 KD patients younger than 1 year of age who were treated at the Korea University Medical Center from January 2001 to July 2006. Of these patients, 16 developed CAA (11.8%). The CAA(+) group had a longer duration of total fever than the CAA(−) group (9.1±3.7 days vs. 6.3±2.0 days, p=0.011), but did not differ in the duration of pre- and post-intravenous gamma-globulin (IVGG) fever. The CAA(+) group had fewer diagnostic symptoms than the CAA(−) group (2.7±1.1 vs. 4.3±1.2, p<0.001). Of the hematological findings, the CAA(+) group only differed from the CAA(−) group in having significantly higher total white blood cell (19.2±6.0 vs. 14.7±4.7 K/mm³, p=0.007) and platelet (462.9±101.0 vs. 383.6±121.1 K/mm³, p=0.014) levels. Multivariable logistic regression analysis showed that the only factors which were significantly associated with the development of CAA were the total number of symptoms (OR=0.493, 95% CI=0.293–0.829, p=0.007) and the duration of total fever (OR=1.405, 95% CI=1.092–1.808, p=0.008). Conclusively, incomplete clinical manifestations and a longer duration of total fever are significantly associated with the development of CAA in KD patients younger than 1 year of age. Therefore, these patients should be monitored for incomplete KD, especially if unexplained fever continues, and treatment to shorten the duration of total fever should be initiated.