A study was performed to assess the feasibility, additional diagnostic value and potential applications of biplane transesophageal echocardiography in neonates, infants and children. One hundred thirty-two consecutive studies were attempted in 111 anesthetized children with congenital heart disease. Longitudinal and transverse planes were compared using 3 methods: (1) separate 7 mm longitudinal and transverse pediatric transducers used sequentially; (2) an experimental 9 x 8 mm biplane pediatric transducer; and (3) a standard adult biplane transducer (12 x 9 or 13 x 9 mm). In all but 1 patient, a probe could be inserted. The longitudinal plane provided superior visualization of both the right and left ventricular outflow tracts, the interatrial septum, the main pulmonary artery, the ascending aorta and the right coronary artery. In 18 patients (16%), the longitudinal plane provided completely new diagnostic information that was not obtained with combined transthoracic and transverse plane transesophageal echocardiography. However, the transverse plane was mandatory for demonstration of the 4-chamber view, short-axis cross sections through the great arteries, the distal right pulmonary artery and bifurcation of the left coronary artery. The longitudinal plane is complementary to the transverse plane, but cannot substitute for it.