Risk factors for coronary artery aneurysms (CAA) related to Kawasaki disease (KD) have included young patient age, particularly <6m. To more broadly determine the relations of age to CAA, a database of 747 patients with KD from 1980 to 1995 was analyzed. Group I included 96 (13%) infants with KD at < 1y; group II, 525 (70%) children aged 1-5y; and group III, 126 (17%) children aged 6+y. There were no differences regarding gender (I-64% male, II-56%, III-61%; p=0.28), but there were fewer non-Caucasians with advancing age (I-45%, II-36%, III-27%; p=0.008). There was no difference in the proportion who met 5+ diagnostic criteria (I-78%, II-78%, III-75%; p=0.75). Fewer older patients received aspirin (I-90%, II-90%, III-75%; p=0.75). Fewer older patients received aspirin (I-90%, II-90%, III-75%; p=0.0001) or gamma globulin (IVGG) (I-56%, II-52%, III-40%; p=0.03). There was no difference in CAA (I-16%, II-10%, III-8%; p=0.17). If coronary artery ectasia was included then there was a decreasing incidence with advancing age (I-38%, II-25%, III-19%; p=0.002). Detailed data available from a smaller subset suggested that younger and older patients had longer mean days from onset to IVGG treatment (I-7.4d, II-6.2d, III-8.3d; p=0.0008). After controlling in logistic regression analysis (n=709) for gender, diagnosis date, number of diagnostic criteria and treatment with aspirin and IVGG, adjusted odds ratios for CAA for age groups (relative to 2y to <4y) were as follows: <6m, 4.89; 6m to<1y, 1.15; 1y to <2y, 1.40; 2y to <4y, 1.00; 4y to <6y, 1.01; 6y to <10y, 0.98; and 10+y, 1.25. Thus, the increased independent risk of CAA appears to be isolated to patients <6m at diagnosis of KD, with treatment delays more likely at the extremes of age.