At the time of initial balloon atrial septostomy a patent ductus arteriosus was found with angiography in 39 of 81 infants with transposition of the great arteries with intact ventricular septum. By angiographic criteria the ductus shunt was considered small in 21 infants and large and significant in 18. In contrast to the usual clinical presentation of neonates with transposition and intact ventricular septum, 12 of these 18 infants with a significant patent ductus arteriosus had only slight cyanosis and 8 presented with tachypnea out of proportion to the degree of cyanosis. Ten of the 18 infants had no continuous murmur, bounding pulses, mid-diastolic rumble or differential cyanosis. Clinically occult narrowing or closure occurred, presumably gradually and relatively late, in six infants. Acute early narrowing or closure, spontaneous (six infants) or surgically produced (three infants), occurred usually within the 1st month of life and was associated with a marked decrease in arterial oxygen saturation in eight infants, often with a rapid clinical deterioration. Persistence of a large patent ductus arteriosus for several months appears to be associated with an increased incidence of early pulmonary vascular disease. Therapeutic considerations for the infant with a large patent ductus arteriosus after initial balloon atrial septostomy include: (1) careful initial follow-up of the infant in clinically stable condition in case the ductus arteriosus should acutely narrow or close and the patient require urgent palliative or corrective surgery; (2) urgent early closure of the ductus in the infant with overt left heart failure with concurrent atrial septectomy or preferably primary corrective surgery; and (3) elective closure of a persistent significant patent ductus arteriosus before age 4 months with concurrent corrective surgery in the infant in clinically stable condition.