Echocardiography was used to evaluate premature infants with large patent ductus arteriosus (PDA) shunts and proved lung disease. A PDA producing large left-to-right shunting also causes left atrial enlargement. Since the left atrial diameter (LAD), measured by ultrasound, normally varies with body size, its value was compared to that of the aortic root diameter (AO) to adjust for body size variations. In those infants with pulmonary disease but no significant PDA, the range of LAD/AO ratios was similar to that of the control group. In patients with a proved large PDA the ratios were significantly higher. The ultrasound ratio returned to normal in all cases following surgical ligation of the PDA.