During a five-and-one-half year period, the transseptal technique was used for left heart catheterization in 37 patients with a variety of congenital heart disease. The age of the patients ranged from four months to 21 years and weights ranged from 6.6 to 65 kg. Access to the left heart cavities and ascending aorta was achieved in all but two patients. There were no complications. Even when used infrequently, the transseptal technique allows rapid and safe entry to the left heart cavities in infants and children.