A 29-year old man was admitted to the intensive care unit after losing consciousness. On physical examination, a loud systolic murmur over the heart was found. Echocardiography revealed narrowing of pulmonary artery with high pressure gradient. Computed tomography of the chest revealed the presence of large tumour localised in the upper anterior mediastinum. Due to the risk of total closure of the pulmonary artery, interventional mediastinotomy was performed and diagnosis of carcinoma embryonale was established. Subsequent chemotherapy (BEP regimen) has brought regression of tumour and significant improvement in haemodynamic parameters (relief of pressure gradient in pulmonary artery). During the second surgery, the resection of all accessible tumour mass together with marginal resection of the right upper lobe was performed. No signs of cardiac or great vessels infiltration was found. Histopathologic examination revealed the necrotic masses and neoplastic foci diagnosed as teratoma immaturum. In a four-month follow-up the patient's condition remained good. The patient is still under the care of both oncological and cardiological specialists. Thus far he has not required further chemotherapy. Holter ECG monitoring revealed no arrhythmia, but the patient is still treated with mexiletine. The patient is planning to return to work.