OBJECTIVE To identify early echocardiographic markers allowing prediction of subsequent symptomatic patent ductus arteriosus (PDA). METHODS One hundred sixteen preterm infants ( < 1500 gm) requiring mechanical ventilation underwent echocardiography at a mean postnatal age of 19 hours (range, 7 to 31 hours). Four potential markers were studied: the left atrial to aortic root ratio, pulsed Doppler signal within the course of the duct (ductal diameter), and the direction of postductal aortic diastolic flow. Subsequent ductal closure or significant patency (if suspected clinically) was confirmed echocardiographically. RESULTS A significant PDA developed in 42 infants (36%). Ductal diameter was the most accurate echocardiographic marker in predicting subsequent significant most accurate echocardiographic marker in predicting subsequent significant PDA. With a ductal diameter of 1.5 mm or greater there were 34 true-positive, 11 false-positive, 63 true-negative, and 8 false-negative results, giving a positive likelihood ratio of 5.5 and a negative likelihood ratio of 0.22 for prediction of development of a PDA requiring treatment. The sensitivity was 81% and the specificity was 85%. Only one infant older than 28 weeks of gestational age had a significant PDA, and limiting the analysis to infants younger than 29 weeks of gestation further improved the predictive accuracy of ductal diameter. The positive likelihood ratio was 8.1 and the negative likelihood ratio was 0.19, with a sensitivity of 83% and a specificity of 90%. CONCLUSIONS Color Doppler measurement of the internal ductal diameter allows early prediction of significant PDA in preterm infants.