We report a case of a 15 year old boy with a cyst attached to the anterior leaflet of mitral valve. At the age of 6 he was submitted for a car diac evaluation due to a heart murmur. On physical exam a soft systolic murmur with loudness of 2–3/6 in Levine scale was found without symptoms of congestion. Electrocardiogram and chest X ray were within normal range. Cardiac echo study (ECHO) revealed a single balloon-like anomaly with small compartments inside. The tumor was localized on the ventricular surface of the anterior leaflet of mi tral valve. Initially inflow and outflow tracts in colour and pulse wave Doppler were normal without any disturbances. During follow up the patient was asymptomatic without episodes of syncope, congestive heart failure or thromboembolic episodes. He was not treated pharmacologically. At the age of 15, in a subsequent ECHO the size of the cyst was up to 20 mm. The inflow was normal without signs of stenosis, while the left ventricle outflow tract revealed mild dynamic stenosis at the end of systole with maximum pressure gradient of 15 mmHg. The child was qualified for surgery, however the parents did not agree for operation due to the high risk of the procedure and high likelihood of a need for mitral prosthesis implantation requiring lifelong anticoagulation