OBJECTIVE: We aimed to compare the severity of coronary artery abnormalities in Kawasaki disease between infants and older children. METHODS: We retrospectively reviewed and compared coronary artery dilation and aneurysm severity in infants <1 year of age with Kawasaki disease at our centre over a 10-year period with that observed in children >/=1 year of age in the Pediatric Heart Network Trial of Pulse Steroid Therapy in Kawasaki Disease. Coronary artery abnormalities were defined by z-scores according to American Heart Association guidelines. RESULTS: Of the 93 infants identified during the study period, 80 were treated with intravenous gamma globulin within the first 10 days of illness and were included for comparison to 170 children >/=1 year of age treated in the same time frame from the Pediatric Heart Network public database. The mean maximum z-score was significantly higher in infants compared with older children (3.37 vs 2.07, p<0.001). A higher incidence of medium and giant aneurysms was observed in infants compared with children >/=1 year of age (11% vs 3% for medium aneurysms, p=0.015; 8% vs <1% for giant aneurysms, p=0.005). CONCLUSIONS: Infants with Kawasaki disease have more severe coronary artery dilation compared with older children, and a higher prevalence of medium and giant aneurysms. Because adverse outcomes are closely linked to the maximal coronary artery diameter in Kawasaki disease, patients diagnosed as infants require very close long-term monitoring for cardiac complications.