A rare isolated double orifice mitral valve (DOMV) was diagnosed in a 77-year-old male patient, being assessed for surgical repair of the ascending aorta. This is a rare congenital abnormality, usually discovered as an incidental finding during investigation of other congenital heart defects. This case shows that a detailed assessment of all cardiac structures is necessary, not only in young patients, but also in the elderly population, to minimise the under-diagnosis of such rare anomalies. The use of 3D transthoracic echocardiography (TTE) has an increasingly significant role in establishing the diagnosis and extending the morphological and functional understanding of the anomaly. LEARNING POINTS Thoroughly assessing all cardiac structures, in accordance with the minimum dataset guidelines for transthoracic echocardiography, ensures not only a comprehensive assessment of the primary indication for the scan, but also improves the detection of concomitant and otherwise unknown lesions. Despite falling under the category of congenital heart defects, several rare anomalies such as DOMV can be present in elderly patients, and the adult echocardiographer should have appropriate knowledge and awareness for detecting these conditions. 3D TTE provides a comprehensive assessment of the morphology of DOMV, over and above the information obtained by 2D imaging.