A left circumflex coronary artery fistula (CAF) is a rare anomaly. This report describes two young children with progressive left coronary artery dilation due to left CAF demonstrated by serial echocardiography. Cardiac catheterization performed for both children confirmed the presence of a markedly tortuous and dilated left circumflex artery, with the CAF draining directly into the right ventricle. Transcatheter closure of the CAF using the Amplatzer vascular plug was successfully accomplished without any complications. A good outcome was achieved. The echocardiography at the 12- and 18-month follow-up visits showed reversion of the coronary artery to normal size.