Balloon atrial septostomy was undertaken under cross sectional echocardiographic control in 63 consecutive infants: in no case was fluoroscopic imaging required. The procedure was performed in the cardiac catheterisation laboratory, ward side room, or at the bedside in the neonatal intensive care unit. Catheterisation via the umbilical vein was attempted in 37 infants aged less than 48 hours old and was successful in 27. No complication was clearly attributable to the procedure though two infants died. A nine day old child died from disseminated intravascular coagulation the day after septostomy by the iliofemoral route and another, aged nine days, died of necrotising enterocolitis which had developed when he was eight days old, after umbilical catheterisation at eight hours. Balloon atrial septostomy is a safe and easy procedure under cross sectional echocardiographic imaging control. Catheterisation via the umbilical vein was safe, easy to perform, and is appropriate in infants aged less than 48 hours.