We performed 138 serial two-dimensional (2-D), M-mode, and Doppler echocardiographic studies to assess the efficacy of intravenous indomethacin (INDO) therapy for patent ductus arteriosus (PDA) in 41 preterm infants. Nine infants expired without sufficient echo data to warrant inclusion in the study. Of the remaining 32 infants, 12 (Group 1) required ligation despite INDO therapy, 18 (Group 2) responded to INDO or spontaneously closed their PDAs, and 2 were transferred to other hospitals with their PDAs still open and were lost to follow-up. The presence of PDA was verified by 2-D echo visualization of the ductus (high parasternal short-axis views) and Doppler demonstration of high velocity ductal flow toward the main pulmonary artery. The ratio of PDA lumen to aortic root inner diameter (Ao) was measured as well. Indication for surgical ligation was failure of the PDA to close after 3 or more courses of INDO. In Group 1, the PDA/Ao ratio was 0.45 +/- 0.10 standard error of the mean (SEM) pre-INDO, and 0.40 +/- 0.12 SEM post-INDO (immediately before ligation). In Group 2, the PDA/Ao ratio was 0.42 +/- 0.11 SEM pre-INDO, and 0.19 +/- 0.14 SEM at the end of INDO course (p less than 0.001), with 12 infants showing no ductal flow by Doppler. Follow-up six months after INDO therapy has shown no residual murmurs or other evidence of PDA in any INDO-closed infant. Five of the 18 infants have undergone repeated echocardiographic studies, all of which were negative for PDA patency or flow.(ABSTRACT TRUNCATED AT 250 WORDS)