The development and regression of the coronary aneurysms in Kawasaki disease was studied with serial two-dimensional echocardiographic (2D echo) examinations. The diameter of the aneurysms at the proximal portions of the left coronary artery was measured on the 2D echo images in ten patients with Kawasaki disease, in whom left coronary aneurysms were found at the acute stage of the illness, and followed by 2D echo for longer than eight months. It was found that coronary aneurysms usually developed during the second week of the illness, reached maximal size at 3-8 weeks, and regressed gradually thereafter. Small aneurysms disappeared in several months, and those of intermediate size regressed in one to two years. Large aneurysms may remain for many years. Mural thrombi within the aneurysms were detected with 2D echo in three patients. They decreased in echodensity and eventually disappeared echographically.