A case of metastatic colonic adenocarcinoma invading the left atrium is reported in a patient with clinical signs of cardiac tamponade. The intracavitary extension of the tumour was clearly demonstrated by contrast enhanced CT. As CT plays an important role in the evaluation of patients with intrathoracic masses, intravenous contrast medium is recommended in those cases with associated clinical symptoms of heart disease or pericardial effusion. Its use may establish the diagnosis of cardiac involvement.