An 11-year-old male castrated Golden Retriever presented to the Emergency Service for vomiting, or possible regurgitation, inappetence, hypersalivation and tachypnea of six days duration. The patient presented to his referring DVM two days prior for these symptoms and was given intravenous fluids, Sucralfate, and Maropitant. His owners also reported that he exhibited hind limb paresis and difficulty posturing to defecate since his visit to the rDVM. On presentation, the dog was bright, alert, and responsive. His physical exam revealed that he was tachypneic, tachycardic, and hyperthermic. When lead on a leash, he was severely exercise intolerant, walked with a rigid forelimb gait, and splaying of the hind limbs was appreciated when standing. The patient was unable to correct his right hind limb from a knuckling position due to neuromuscular deficits, and there were no abnormalities on neurologic exam. Three view thoracic radiographs were performed that showed the patient had a megaesophagus. Differentials for megaesophagus include neurologic disorders, neuromuscular diseases, such as Myasthenia Gravis, toxins, endocrinopathies, and idiopathic causes. Additional tests were performed, including a total T4, baseline cortisol level, and an abdominal ultrasound. Results of these tests were all within normal limits. Myasthenia Gravis was suspected, and a Tensilon response test was performed to confirm the presumed diagnosis. The patient had a positive response to the Tensilon test, and was started on Mestinon (Pyridostigmine bromide), an anticholinesterase inhibitor. The patient’s blood work was submitted for antibody titers against acetylcholine receptors. The result was positive for antibodies, confirming that the patient’s symptoms were due to Myasthenia Gravis. On a recheck examination, repeat radiographs revealed a cranial thoracic mass, and the CT results were highly suggestive of a thymoma. It is hypothesized that the Myasthenia Gravis is a sequela to the thymoma, and the patient will undergo surgery to remove the probable thymoma.