A new technique for transseptal left heart catheterization utilizing the Transseptal Introducer Set in 520 patients is described in detail. The age range of these patients was 3 months to 40 years with 30% of the patients less than 2 years old and 28% less than 10 kilograms in weight. The safety and advantages of the technique are emphasized. The introducer set and new technique make transseptal left heart catheterization a safe, versatile, and dependable approach to all areas of the left heart, particularly in infants and children with complex congenital heart lesions.