DISCUSSION: Lung cancer is the leading cause of cancer-related deaths in the US (27% of all cancer deaths). Squamous cell carcinoma, commonly seen in smokers, arise frequently in the proximal bronchi and tend to cause bronchial obstruction, atelectasis, pneumonia or pericardial effusions. Few cases have documented the occurrence of arrhythmia as the first manifestation of a tumor invading atrial structures. The most dramatic presentation is a sudden conduction abnormality with various degrees of heart block. Atrial fibrillation episodes are triggered by ectopic beats from fibers extending from the left atrium into the pulmonary veins. Tumor cells infiltrate the myocardium creating a pulmonary vein sleeve, which is interdigitated by smooth muscle and fibrous tissue, thus creating a chance for micro-reentry. Mediastinal tumors can also result in mechanical stretching of the pulmonary vein(s) and atrial fibers resulting in tachycardia. In our case enhanced CT revealed the presence of a lesion encircling/narrowing the inferior pulmonary veins. We propose that his presentation could be explained by a reentry, presence of pericardial effusion, and/or as a consequence of the compression of the heart by the growing tumor.