Transesophageal echocardiography was attempted in 59 pediatric patients with congenital heart disease in the operating room (n = 33) or during heart catheterization or in the intensive care unit (n = 26). Six different commercially available transducers were used with diameters ranging from 7 to 15 mm. Age ranged from 1 day to 16 years, and body weight ranged from 3.7 to 65 kg. Objectives of the study were to determine (1) minimum body weight in which transesophageal echocardiography with various probes is possible, (2) additional diagnostic value, and (3) potential applications. In three cases (one surgical and two nonsurgical) the probe could not be inserted. Minimum body weight was 17 to 20 kg for probes with a diameter of greater than or equal to 13 mm and approximately 12 kg for the 11 mm probes. A 7 mm probe, on the other hand, could be inserted easily in all patients (including a neonate) in whom transesophageal echocardiography was attempted. In 11 of 56 patients, additional diagnostic information was obtained. Thus, transesophageal echocardiography is feasible in the pediatric age group provided that special probes are used in small children. Additional diagnostic information can be obtained, and the technique is of value during cardiac surgery or balloon interventions for evaluation of the efficacy of the procedure and for monitoring ventricular function.