Abstract Objective: To determine coronary artery diameter change before and after an intracoronary infusion of isosorbide dinitrate in patients who have had Kawasaki disease, we performed coronary angiography in 188 such patients. Patients and methods: Four patient groups were studied. Groups 1 to 3 consisted of patients with Kawasaki disease: group 1 (the “normal” group; n=65) had no coronary artery lesions; patients in group 2 had regression to normal of a coronary artery aneurysm after Kawasaki disease and were divided, according to the time since onset, into two subgroups, group 2-a (n=20) being at an early stage after regression to normal (followed for 1.7±0.7 years) and group 2-b (n=34) being at a later stage after regression (followed for 8.7±2.9 years); and group 3 (the “abnormal” group; n=25) had persistent coronary artery aneurysm or stenosis or both. Group 4, the control group (n=44), consisted of patients without Kawasaki disease who had congenital heart disease with normal coronary arteries. The coronary artery diameter change was calculated by the following formula: Percentage of change=(Diameter after infusion — Diameter before infusion)/Diameter before infusion×100. Results: The percentages of change in the coronary artery diameter after isosorblde dinitrate infusion were 16.2%±13.4% (normal group), 11.3%±7.2% (early-regression group), 7.8%±8.3% (late-regression group), 6.8%±7.8% (abnormal group), and 15.2%±11.8% (control group). In the abnormal group, there was significantly poorer dilation than in the control and normal groups. The late-regression group also had significantly less change in diameter than did the control and normal groups (p<0.05). Conclusions: These data suggest (1) that the coronary artery after Kawasaki disease becomes stiff not only in patients with persistent abnormal lesions but also in those with regressed aneurysms and (2) that stiffness of the coronary artery may be a risk factor for atherosclerosis.