OBJECTIVE To identify risk factors for severe sequelae, analyze disease characteristics, and assess efficacy of intravenously administered immune globulin (IVGG) therapy in infants less than 12 months of age with Kawasaki disease. METHODS Retrospective chart review of children less than 12 months of age with Kawasaki disease between 1980 and 1993. RESULTS Of 443 patients with Kawasaki disease, 57 (13%) were less than 1 year of age, including 14 (3%) less than 6 months. Age at onset was a predictor of the development of coronary artery aneurysms (CAA) and of giant (> 8 mm) aneurysms: 11 (79%) of 14 children < 6 months and 17 (44%) of 39 children 6 to 12 months of age acquired CAA (p = 0.06), and 5 (37%) of 14 children < 6 months and 2 (5%) of 39 children 6 to 12 months of age acquired giant CAA (p < 0.01). No specific clinical or laboratory features predicted the development of CAA, which was found in 7 (29%) of 24 patients treated with IVGG by illness day 10 and in 21 (73%) of 29 patients treated after day 10 or never treated with IVGG (p < 0.01). Only 1 (4%) of 24 patients treated by day 10 but 6 (21%) of 29 children treated after day 10 or never treated with IVGG acquired giant CAA (p = 0.01). Persistent (> 1 year) CAA were present in 4 (17%) of 24 IVGG-treated children by day 10 and in 14 (48%) of 29 children not treated by day 10 or never treated with IVGG (p < 0.025). There was no difference in outcome if IVGG was given by illness day 7 or on illness days 8 to 10. CONCLUSIONS Patients with Kawasaki disease less than 6 months of age are at particularly increased risk of having CAA and giant CAA. Therapy with IVGG, given by illness day 10, is associated with substantial reduction in the frequency of CAA and giant CAA in this high-risk population.