Cor Triatriatum Sinister (CTS) represents a rare congenital cardiac defect characterized by the presence of a fibro-muscular membrane separating the left atrium into two cavities. During adulthood CTS may be asymptomatic and incidentally found during imaging diagnostic workflow. Bidimensional (2D) transthoracic echocardiography (TTE) is considered the first-line imaging technique for the evaluation of CTS. Cardiac magnetic resonance (CMR) and transoesophageal echocardiography (TEE) are advanced diagnostic tools, usually required to obtain additional data. When these diagnostic techniques are contraindicated or not available, three-dimensional (3D) TTE may provide useful information which may help in the morphological assessment of CTS. Moreover, this widespread technique is safe and frequently available in the echocardiography laboratories, although the acoustic window and the training of the operator may be limiting factors. With this case report we sought to emphasize the usefulness of this technique as a valid alternative to CMR and TEE in selected settings.  Kulcsszavak: Cor triatriatum sinister, 3D transthoracic echocardiography