• Factors potentially associated with the formation or prevention of coronary artery aneurysms were investigated in 77 children with Kawasaki disease. The Patients were divided into two groups. Group A consisted of those who developed coronary artery aneurysms and group B consisted of those who did not. The patients who developed aneurysms had a significantly longer duration of fever and a significantly lower minimum hemoglobin concentration than those who did not develop aneurysms. In patients who developed aneurysms, aspirin therapy was begun significantly later in the course of the illness compared with those who did not develop aneurysms. We suggest that the timing of the initiation of aspirin therapy may be important in the prevention of coronary artery aneurysms in patients with Kawasaki disease. We believe that it may be important to establish the diagnosis of Kawasaki disease as early as possible and to institute aspirin therapy at an appropriate dosage.