OBJECTIVES: To assess the effectiveness of atrial septostomy by percutaneous balloon dilatation in patients with congenital heart defects or primary pulmonary hypertension. PATIENTS AND DESIGN: Twenty three patients (15 boys, eight girls; aged 10 days to 10 years; 17 with congenital heart defects and six with primary pulmonary hypertension), all haemodynamically unstable under optimal medical treatment, underwent atrial septostomy by percutaneous balloon dilatation. INTERVENTIONS: The balloon catheter entered the left atrium through a patent foramen ovale (n = 14) or via transseptal puncture in cases with an intact atrial septum (n = 9). The size of the balloons used ranged from 13 to 18 mm. RESULTS: There were no complications. The interatrial communication (mm) increased (P < 0.05) after dilatation and remained unchanged (P = NS) during a 16.6 (13.8) month follow up (2 (1.7) v 8.8 (1.4) v 8.2 (1.1), respectively). Transatrial gradient (mm Hg) fell and arterial oxygenation (%) improved both in patients with transposition (6.3 (0.8) v 0.8 (1) (P = 0.0001) and 40.6 (4.2) v 76.5 (4.8) (P = 0.0001), respectively) and in those with mitral atresia (13.4 (1.9) v 2 (1.4) (P = 0.0001) and 77.1 (3.9) v 81.5 (4.2) (P = 0.008), respectively). There were two failures, one early and one late, both in the group of patients with mitral atresia or stenosis. A decrease in arterial oxygenation (94.8 (1.5) v 83 (2.4), P = 0.004) and an increase in left atrial pressure (6.8 (0.9) v 8.3 (1.2), P = 0.02) and cardiac index (2.3 (0.2) v 3.1 (0.2) l/min/m2, P = 0.002) was observed in patients with primary pulmonary hypertension. CONCLUSIONS: Percutaneous balloon dilatation is an effective and safe procedure for creating an adequate interatrial communication that can be used as an alternative to blade septostomy.