Pericardial effusion and a mass compressing the left atrium externally were detected using transthoracic echocardiography in a 24 year old woman who had been treated due to a kind of breast cancer called cystosarcoma phyllodes. Symptomatic relief was achieved with pericardiosynthesis. An echocardiographic examination should be performed to patients having dispnea and a story of cancer. Pericardial effusion is detected more commonly however, a mass in or out of the cardiac chambers, or compressing these chambers might be demonstrated like in our case.