A 10 year old female spayed Shih-Tzu presented to the Oncology Service at the Cornell University Hospital for Animals for workup of a previous episode of dyspnea. On examination, the dog appeared clinically healthy. Thoracic radiographs revealed mild pleural effusion. An echocardiogram revealed moderate pericardial effusion. Cytologic analysis of a suspected mixed pericardial and peritoneal fluid sample revealed an exudative effusion with atypical mesothelial cells. Based on initial diagnostic tests, a presumptive diagnosis of mesothelioma was made. The patient was managed conservatively with a plan to perform palliative thoracocentesis as clinical signs recurred. This case report discusses potential causes of bicavitary effusion in the patient and provides an overview of the presumptive diagnosis, mesothelioma.