To construct a predictive instrument for developing coronary artery abnormalities in patients with acute Kawasaki disease treated with aspirin and intravenous gamma globulin within the first 10 days of illness, data available from a multicenter database of patients with acute Kawasaki disease were analyzed. A development data set (n = 212) was used to construct a sequential risk classification instrument based on easily measured baseline laboratory test results and temperature. The instrument was then validated in 3 test data sets (n = 192, 264, and 92, respectively). Risk factors used in the sequential classification instrument included baseline neutrophil and band counts, hemoglobin concentration, platelet count, and temperature on the day after infusion of intravenous gamma globulin. In the development data set, the instrument classified 123 of 212 patients (58%) as low risk; none developed coronary artery abnormalities. Among 89 patients classified as high risk, 3 of 36 female (8.3%) and 9 of 53 male patients (17.0%) developed coronary artery abnormalities. The instrument performed similarly in the 3 test data sets; no patient in any data set classified as low risk developed coronary artery abnormalities. This simple instrument allows the clinician to identify within 1 day of treatment low-risk children in whom extensive and frequent cardiac testing may be unnecessary, as well as high-risk children who require closer monitoring and may be candidates for additional therapies.