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As part of a multicenter collaborative study, M-mode echocardiograms were obtained shortly after birth on 3,559 premature infants with birth weight less than or equal to 1,750 g. Of these infants, 1,496 did not develop a cardiac murmur or other signs of a "hemodynamically significant" patent ductus arteriosus (PDA). Echocardiographic parameters from this "normal" group were compared with results obtained from 415 infants in whom PDA was diagnosed on the basis of clinical findings alone, irrespective of the echocardiogram (PDA group). The best discrimination between the two groups, when considering a single parameter, was provided by a left atrial to aortic root ratio (LA/AO) of 1.40, a left ventricular to aortic root ratio (LV/AO) of 2.10, or a left ventricular systolic time interval ratio (LPEP/LVET) of 0.27. Multivariate analysis demonstrated that better separation between the two groups occurred when the left atrial to aortic root ratio and the left ventricular systolic time interval ratio were considered jointly. Because of a large degree of overlap of all echocardiographic variables between the normal group and the group with PDA, the echocardiogram alone was not a good indicator of PDA. However, when used in conjunction with a priori estimates of the probability of PDA (based, for example, on birth weight and degree of respiratory disease), use of echocardiographic data was found to improve the detection of hemodynamically significant patent ductus arteriosus in premature infants.