Between December 1982 and April 1991, balloon atrial septostomy was performed in the intensive care unit under echocardiographic control in 60 neonates. Of the patients, 58 had complete transposition. Two patients had double outlet right ventricle with a sub-pulmonary ventricular septal defect. Associated lesions included a patent arterial duct in 19 patients, ventricular septal defect in nine, obstruction of the left ventricular outflow tract in six, aortic coarctation in two and tricuspid atresia in one. The mean age at septostomy was four days (range 4 hours - 25 days) and the mean weight 3.19 kg (range 1.17–4.25 kg). In 39 (65%) patients, an infusion of prostaglandin was in progress prior to the septostomy and 22 (37%) were being ventilated. Standard subcostal four-chamber echocardiographic views were used to show the atrial septum and to guide the catheter used for septostomy. Venous access was obtained via the femoral vein in 43 (by percutaneous puncture in 40 and by cutdown in three) and the umbilical vein in 17. Transient atrial arrhythmias were common during the septostomy but no acute hemodynamic disturbances or deaths occurred during the procedure. The size of the atrial septal defect as measured by echocardiography after the septostomy ranged from three to 12 mm in diameter. In only one patient was this inadequate. Three (5%) patients died between two and 10 days after the septostomy, two due to necrotizing enterocolitis and one from persistent hypoxemia. One patient had a cerebral thrombosis and convulsions immediately after the septostomy but made a good neurological recovery. Corrective surgery was performed in 52 (86.6%), two (3.3%) had palliative surgery and two were considered unsuitable for total correction, of whom one has died. One patient died whilst awaiting correction. We conclude that balloon atrial septostomy using echocardiographic guidance can be safely and effectively performed in the intensive care unit.