Gamma globulin administered in a single dose of 1 g/kg of body weight intravenously caused prompt clinical improvement in 27 of 32 consecutive children with Kawasaki disease treated by the 12th day of illness. Response was equally good for the 20 children treated in the first week and the 12 treated in the second week. Fever and clinical signs abated within the first day after treatment, the mean white blood cell count normalized by 48 hours, and the sedimentation rate continued to be elevated for about 2 weeks, while the platelet count rose during the first 2 weeks after treatment and returned to normal approximately 1 month after treatment. Five children with incomplete relief needed more than the single dose before resolution of signs and symptoms occurred. Coronary aneurysms in 2 patients before treatment regressed by 2 weeks. No patient developed coronary aneurysms. No child had sequelae of Kawasaki disease at a follow-up of 2 to 31 months. We believe that although this was a one-arm, uncontrolled pilot study, the results suggest that this protocol provides a safe, flexible, and effective treatment for acute Kawasaki disease.