This study was designed to evaluate the relative diagnostic values of transthoracic (TTE) and transesophageal (TEE) echocardiography in the assessment of congenitally corrected transposition of the great arteries in adult patients. Twelve patients (mean age 29 years, range 21 to 39 years) with congenitally corrected transposition of the great arteries underwent both TTE and TEE examinations to assess this complex cardiac lesion. Of the 12 patients evaluated, situs solitus and inversus were present in 8 and in 4 patients, respectively. TTE correctly identified atrial situs in only 10 patients, whereas TEE, directly evaluating the morphologic features of either appendage, correctly determined situs in every patient. In all 11 patients with intact inlet ventricular septum, the spatial relationship between the septal leaflets of atrioventricular valves was correctly evaluated by both techniques. However, the chordal attachments of both valves were clearly elucidated by TEE in all patients, whereas TTE could obtain images of these in only three patients. TTE was able to evaluate the discordant connection between the right ventricle and the anterior vessel (aorta) in 10 patients, whereas the connection between the left ventricle and the posterior vessel was clearly shown only in 7 patients. Transesophageal longitudinal planes better elucidated these two discordances in all patients irrespective of the position of the heart in the chest and atrial situs. Four patients had an associated ventricular septal defect (inlet defect in one, perimembranous in two, and muscular in one); the inlet defect was unrestrictive and could be easily detected by either imaging technique, whereas the membranous was detected by TTE and by the horizontal transesophageal planes; the muscular defect was recognized only by TTE. Three patients had an associated pulmonary stenosis; Doppler transthoracic echocardiography showed a left outflow peak gradient of 100 mm Hg in two patients and of 80 mm Hg in one but failed to adequately assess the morphologic features of the stenosis, whose features were clearly visualized by transesophageal longitudinal planes in all patients. In conclusion, in our experience TEE is superior to transthoracic imaging in studying congenitally corrected transposition of the great arteries in adult patients; the horizontal plane is best suited to the evaluation of atrial situs and the atrioventricular junction, whereas the longitudinal plane is most valuable in the study of the morphologic features of the ventriculoarterial connections. These findings should be equally applicable to multiplane transesophageal studies.