Over a 5 yr period, from October 1983 to September 1988, transseptal left heart catheterisation, using Mullins long-sheath technique, was performed in 217 infants, children, and young adults, with various forms of congenital or acquired heart disease. The mean age was 50 +/- 45 months, (range 1 mo to 21 yr). The procedure was successful in 215 (99%) patients. Pericardial puncture occurred in three patients (1.4%), though without tamponade or sequelae. There was no mortality or morbidity. Transseptal left heart catheterisation is a safe procedure for studying the left side of the heart. In addition, and of increasing importance, it allows the performance of interventional procedures on the left side of the heart and avoids the use of retrograde catheterisation through the femoral arteries.