BACKGROUND Left atrial appendage (LAA) ostial stenosis is a rare cardiac condition usually identified by transesophageal echocardiography (TEE). LAA ostial stenosis can be classified into 2 types: one is idiopathic and the other is a complication after incomplete surgical LAA ligation. The former one is even rarer and only 12 cases have previously been reported. CASE REPORT An asymptomatic young woman was found at a medical checkup to have an abnormal jet signal in the left atrium on transthoracic echocardiography. TEE revealed that the abnormal signal originated from a narrowed orifice of the LAA. There were no other cardiac abnormalities in this woman. Unlike the previously reported cases, our case had LAA ostial stenosis detected by transthoracic echocardiography but not by TEE. CONCLUSIONS We describe a case of anatomic narrowing of the LAA orifice in a young woman. This was a first-ever case of idiopathic LAA ostial stenosis that was detected by transthoracic echocardiography. Because of the very small number of such cases reported, it remains unknown whether anticoagulation is required to prevent intra-atrial thrombus formation.