We studied the effects of immune globulin and aspirin versus aspirin alone on platelet count, platelet activation, and factor-mediated coagulation in patients with Kawasaki syndrome. Coagulation tests were performed on the day of admission to the study and 4 to 6 days later. Twenty-three patients were enrolled; 12 received immune globulin intravenously plus aspirin, and 11 received aspirin alone. At initiation of the study the groups were comparable with regard to age, sex, race, and time from onset of illness to study entry. Coagulation values were similar at entry with the exception that the aspirin group had a geometric mean platelet count that was higher than the platelet count in the aspirin-immune globulin group (p = 0.02). Four days after entry there were no significant differences between the two groups in any coagulation studies. Although the immune globulin preparation used has been effective in reducing the prevalence of coronary artery aneurysms, it appears to have no early effect on reduction of platelet activation or other measures of coagulopathy. The mechanism of action of immune globulin in patients with Kawasaki syndrome remains to be elucidated.