Two-dimensional echocardiograms were reviewed in six patients with cor triatriatum (CTA) and six patients with total anomalous pulmonary venous connection to the coronary sinus (TAP) to characterize the echocardiographic features of distinguishing both diseases. Patients' ages ranged from nine days to 25 months in CTA, and from one to 11 months in TAP. The echocardiographic findings of CTA included three findings which were previously reported; the "double arch appearance" of the intra-atrial membrane, a "recess" which developed between the posterior wall of the aorta and the membrane, and the small size of the "recess", and two additional peculiar findings: the direction and echo density of the intra-atrial membrane. The "double arch appearance" of the intra-atrial membrane could not be detected in any of the CAT nor in TAP cases, except in one TAP patient who had a drainage vein obstruction. The "recess" was found in all six TAP patients, but it was recognized only in three of six patients with CTA. Among six patients with TAP, the size of a "recess" was small in two and large in one patient. Among three patients with CAT who had a "recess", the size was small in two and large in one patient: Thus, the size had little to do with differentiating CTA and TAP. The intra-atrial membrane ran parallel with the mitral valve ring in all patients with CTA except one in whom the membrane could not be identified because of the presence of endocardial cushion defect and a round membrane. On the contrary, the membrane ran parallel with the posterior wall of the common pulmonary venous chamber in all patients with TAP. The echo of the membrane was of low density in all patients with CAT and in three of six patients with TAP. The "double arch appearance" of intra-atrial membrane, the presence of a "recess", and the size of the "recess" were not characteristic echocardiographic features which could distinguish CAT from TAP. However, the direction of the membrane was useful as a means of distinguishing two anomalies. The intra-atrial membrane runs parallel with the posterior wall of the common pulmonary venous chamber in TAP, and with the mitral valve in CTA. The echo density of the membrane could not clearly differentiate CAT from TAP, but it was of low density in all patients with CTA.