A 10-year-old female spayed Labrador Retriever presented to the Emergency Service at Cornell University Hospital for Animals in December of 2014 for further evaluation of a cranial mediastinal mass, diagnosed on thoracic radiographs by the primary care veterinarian. At the time of presentation, the owners reported weight loss and difficulty breathing for a one-week period. On presentation to the Emergency Service, the patient was bright and alert, with decreased heart and lung sounds on auscultation of the left cranial thorax. A grade IV/VI murmur was noted on cardiac auscultation. The remainder of the physical exam was unremarkable, and blood work revealed no significant abnormalities. The patient was transferred to the Oncology Service for further diagnostic procedures and staging. Abdominal ultrasound revealed multiple hypoechoic splenic nodules, a hypoechoic nodule in the region of the gastric lymph node, and mild generalized hepatomegaly. Ultrasound-guided splenic aspirates were non-diagnostic on cytology. Similarly, ultrasound-guided aspirates of the cranial mediastinal mass were non-diagnostic on cytology, consisting of low numbers of small and intermediate lymphocytes. Differentials for this patient included thymoma, thymic lymphoma, histiocytic sarcoma, and thymic cyst. CT confirmed a cranial mediastinal mass projecting predominantly into the left thoracic region, composed of approximately 90% fluid and causing caudal displacement of the cardiac silhouette and cranial lung lobes. Although there was no evidence of invasion into surrounding structures, the mass was in direct contact with the pericardium, aortic arch, pulmonary trunk, cranial vena cava, left brachiocephalic trunk, and internal thoracic vessels. A median sternal thoracotomy was performed and a 15 x 10 x 10cm mass was carefully dissected from the surrounding soft tissue structures. Due to close adherence to these structures, a partial pericardectomy was performed, and a small amount of tissue remained adhered to the left phrenic nerve after debulking. The patient’s recovery from anesthesia was uneventful, however a decrease in packed cell volume overnight necessitated a transfusion. The patient was discharged to the owners three days later. Histopathology of the mass revealed a thymoma, a neoplasm of thymic epithelial cells. With surgery alone, the median survival rate for patients that undergo a thymectomy ranges from one to three years. Due to the remnant microscopic disease left behind, continued monitoring with thoracic radiographs every three to four months was recommended.