The morphology of the atrial appendages was examined in 1842 specimen hearts from patients with congenital lesions. The external and internal features that permitted the identification of the right and left appendages were studied in detail in one tenth of the hearts. These results were compared with a similar analysis of 25 normal hearts. This study showed that criteria for identification of right and left appendages were reliable. Application of these criteria to the overall collection identified the usual arrangement in 1776 (97%) hearts, a mirror image arrangement in eight (0.4%); left atrial isomerism in 22 (1.2%); and right atrial isomerism in 36 (1.9%). Fourteen (0.81%) had juxtaposed atrial appendages (13 with usual arrangement and one with left isomerism). This did not interfere with identification of the left and right atria on the basis of appendage morphology. In only two cases did the determination by atrial morphology produce a result that was inconsistent with the arrangement of the other thoracoabdominal organs. Further examination of the atria in these showed a mistake had been made in the initial assessment. The atrial arrangement can be accurately determined by the morphology of the atrial appendages.