An 8 year old spayed female Golden Retriever was referred to the Triage Service at the Cornell University Hospital for Animals for evaluation of pericardial effusion. Echocardiography demonstrated pericardial effusion with cardiac tamponade. Pericardiocentesis was performed with cytological evaluation indicating mesothelioma. Mild pericardial effusion was evident one week later and thorascopic partial pericardectomy was performed. Biopsy of the pericardium was equivocal with differing opinions from the pathologists (mesothelioma versus chronic active pericarditis). Chest radiographs were taken four months after the surgery and revealed no signs of pleural or pericardial effusion. The dog has remained clinically normal post-operatively.