Transcatheter closure of atrial septal defect (ASD) was accomplished in 10 of 11 patients aged 13 months to 46 years (weight range 11 to 77 kg). Transesophageal echocardiography (TEE) was used simultaneously with fluoroscopic imaging in 4 of these patients aged 4.5 to 46 years (weight range 19 to 77 kg). TEE was used to ascertain defect size, position and number of defects and to ascertain appropriate seating of the defect occluder within the atrial defect. In 2 patients TEE-assisted transcatheter ASD closure was accomplished after previous attempts at transcatheter ASD closure, unaided by TEE, had been unsuccessful. The only unsuccessful ASD closure procedure occurred in the smallest patient in the series (an 11-kg 13-month-old), a child who was too small to undergo TEE using our 11-mm diameter endoscopic probe. The concomitant use of TEE with fluoroscopic imaging provides information that is unique and complementary and may improve the efficacy and safety of the transcatheter technique for ASD closure. The recent availability of a 7-mm diameter TEE probe will extend the use of TEE into the infant age group and may decrease the discomfort and potential morbidity of TEE in older patients.