Thirty preterm infants (gestational age 26 to 30 weeks) were investigated by cross sectional echocardiography using a 5 or 7.5 MHz transducer positioned in the suprasternal notch or the left subclavicular position to enable visualisation of the aortic arch, main pulmonary artery, left pulmonary artery, and ductus arteriosus. Each infant was investigated on at least one occasion during both the first and second weeks of life and when possible between the ages of 26 and 31 days. There was prolonged patency of the ductus arteriosus during the first two weeks of life in all infants, and complete closure of the ductus arteriosus occurred only in four patients, all of whom had reached a postconceptional age of 32 to 34 weeks. Eighteen infants received intravenous indomethacin between the age of 5 and 10 days. In these patients ductal narrowing did not occur until a maximum postconceptional age of 29 weeks.