Two-dimensional echocardiography (2-D echo) was performed before coronary angiography in 213 patients (pts), 4 months to 12 years old, with a history of Kawasaki disease (MCLS). In 10 pts, 2-D echo was performed almost everyday during acute phase. In 23 pts, coronary artery aneurysms were followed up for more than 6 months by 2-D echo. In 32 of 213 pts, the presence of coronary artery aneurysms (9 pts with left, 4 pts with right and 19 pts with both coronary arteries) were proved by coronary angiography. The correct diagnosis was achieved prospectively by 2-D echo in 26 of 32 pts with coronary artery aneurysm (in 24 of 28 pts with left and 15 of 23 pts with right coronary artery aneurysm). In all 10 pts with acute phase of MCLS, an increased echocardiographic density around the coronary artery and of the coronary artery itself continued from the 4th or 6th day to the 10th day or near the second month of the illness. The findings were considered due to acute perivasculitis and vasculitis in the coronary artery. In 5 of 10 pts, the dilatation of coronary artery was demonstrated on about 6th day of the illness and in 3 pts was transient, but in other 2 pts the dilated coronary arteries grew up aneurysms on the 9th and 13th day of the illness. In 23 pts with coronary artery aneurysm followed up by 2-D echo for more than 6 months, 18 pts were started to observe within 6 months and 5 pts beyond 1 year after the onset of illness. In the former, the size of aneurysm was markedly reduced in 7, slightly reduced in 6, and did not change in 5 pts. On the other hand, it did not change in all the 5 pts of the latter. This study suggests that 2-D echo is very useful to diagnose noninvasively coronary artery aneurysm in pts with the history of MCLS and to detect and follow-up it in acute phase.