This study was performed to assess the prevalence of Chiari's network in patients undergoing transesophageal echocardiography and to determine whether this anomaly is associated with other cardiac lesions or is characterized by typical clinical findings. Chiari's network is a congenital remnant of the right valve of the sinus venosus. It has been found in 1.3% to 4% of autopsy studies and is believed to be of little clinical consequence. Video recordings of 1,436 consecutive adult patients evaluated by transesophageal echocardiography over a 30-month period were reviewed for the presence of Chiari's network. Echocardiographic contrast studies had been performed in all patients with Chiari's network and were compared with those of 160 consecutive patients without a Chiari net, serving as a control group. Chiari's network was present in 29 of 1,436 patients (prevalence 2%). A frequently associated finding was a patent foramen ovale in 24 (83%) of the 29 patients with Chiari's network versus 44 (28%) of 160 control patients (p < 0.001). Intense right-to-left shunting occurred significantly more often in patients with Chiari's network than in control patients (16 [55%] of 29 patients vs. 19 [12%] of 160 control patients, p < 0.001). Another frequent association was an atrial septal aneurysm in 7 (24%) of 29 patients. The indication for transesophageal echocardiography was a suspected cardiac source of arterial embolism in 24 (83%) of 29 patients with a Chiari net, 13 of whom (54%) had recurrent embolic events. Chiari's network was significantly more common in patients with unexplained arterial embolism than in patients evaluated for other indications (24 [4.6%] of 522 patients vs. 5 [0.5%] of 914 patients, p < 0.001). Potential causes for arterial embolism were present in 9 of the 24 patients with a Chiari net and embolic events (atrial septal aneurysm in 7, cerebrovascular lesion in 2). In 15 (62%) of 24 patients only a patent foramen ovale could be identified. Three patients had deep venous thrombosis and pulmonary embolism at the time of arterial embolism; none had a thrombus detected within the network. In patients undergoing transesophageal echocardiography, the prevalence of Chiari's network was 2%, which is consistent with autopsy studies. By maintaining an embryonic right atrial flow pattern into adult life and directing the blood from the inferior vena cava preferentially toward the interatrial septum, Chiari's network may favor persistence of a patent foramen ovale and formation of an atrial septal aneurysm and facilitate paradoxic embolism.