To assess the anatomy of the left atrium and mitral plane after heart transplantation, we performed a transesophageal echocardiographic study to 37 consecutive transplant patients. After heart transplantation no patient was under anticoagulant treatment and no case of atrial fibrillation was documented. The transesophageal approach allowed us to measure the left atrial free wall suture which was: less than 15 mm in 14 patients, between 15 and 25 mm in 16 patients, and more than 25 mm in 7 patients. In those patients with a left atrial free wall suture greater than 15 mm, blood flow turbulences within the "niche" underneath the protruding suture as well as blood flow acceleration at the rim of that suture were noted. In 4 patients a "pseudoaneurysm" of the interatrial septum was observed. Two patients had mitral valve prolapse. Mitral regurgitation was noted in 17 patients (46%) by color Doppler transesophageal echocardiography and graded as mild in 15 patients and moderate in 2 cases. In 16 patients (43%) spontaneous echo contrast within the left atrium was detected by transesophageal echocardiography. Both major and minor axis as well as left atrial area in patients with and without dynamic echoes were, respectively: 72.5 +/- 12.2 mm vs 56.9 +/- 5.9 mm (p < 0.001), 48.3 +/- 7.1 mm vs 39 +/- 7.9 mm (p < 0.001), and 35.4 +/- 7.1 cm2 vs 24.4 +/- 5.2 cm2 (p < 0.001). Atrial thrombi were not detected. After a mean follow-up of 15 +/- 10.7 months there was no arterial thromboembolism in patients with spontaneous contrast.(ABSTRACT TRUNCATED AT 250 WORDS)