Because intravenously administered immune globulin (IVIG) is effective in reducing the incidence of coronary artery aneurysms in Kawasaki syndrome when given at a dose of 400 mg/kg daily for 4 days, we undertook a multicenter clinical trial comparing two dosage regimens of IVIG. Patients were randomly assigned to receive IVIG at either 400 mg/kg daily for 4 days (22 patients) or 1 gm/kg as a single dose (22 patients). All patients received aspirin therapy, and all were enrolled within 7 days of onset of fever. The presence of coronary artery aneurysms was evaluated by means of two-dimensional echocardiography before infusion; at days 4 to 6, 14 to 21, and 42 to 49 after infusion; and at 1 year. Coronary artery aneurysms were detected in 3 of the 44 patients, including one patient receiving 400 mg/kg and two patients receiving 1 gm/kg (p value not significant). No giant aneurysms were detected. No major side effects occurred with either dosage regimen. Patients receiving the 1 gm/kg dose had a faster resolution of fever and were discharged from the hospital approximately 1 day sooner than the 400 mg/kg group (p = 0.01). Although the relatively small sample size in this trial does not allow for a more definitive statement regarding the occurrence of coronary artery aneurysms, it appears that the 1 gm/kg dose is associated with a more rapid clinical improvement and a shorter hospital stay.