An eight-year-old male castrated mixed breed dog presented to Cornell University Hospital for Animals’ Internal Medicine service in July 2012 for further evaluation of hypoglycemia found by the referring veterinarian. The patient had an approximately one year history of intermittent seizure-like activity, during which time his head would twitch, teeth chatter, and occasionally lose footing. Initial findings by the primary veterinarian revealed a significant hypoglycemia with inappropriately high serum insulin concentration. After ruling out hypoadrenocorticism as one of the causes with an ACTH stimulation test, referral was made to Cornell for further diagnostics. On presentation, the dog was bright and alert. All vital parameters were within normal limits and no abnormalities were found on physical exam. Blood was collected and submitted for a complete blood count (CBC), chemistry panel, baseline insulin level, and fasting bile acids. Results showed a significant hypoglycemia (32 g/dL; normal 60-120) with a concurrent inappropriately elevated serum insulin level (21.18 uIU/mL; normal 5-25). All other parameters on CBC and chemistry panel were within normal ranges. Fasting bile acids was also normal (4 umol/L; normal 0-13). Thoracic radiographs were performed and showed no evidence of metastasis or primary lung neoplasia. An abdominal ultrasound followed, which showed no evidence of pancreatic or any other abdominal neoplasia. Major differential diagnoses (hepatic disease, hypoadrenocorticism, non-pancreatic beta cell tumor causing a paraneoplastic syndrome, and sepsis), were able to be ruled out based on results from blood work and imaging. This left the most likely diagnosis to be an insulinoma. Discussion of treatment options followed, and medical management for a presumptive insulinoma was pursued. The patient was started on 0.5 mg/kg of prednisone daily. The following paper will discuss common clinical presentations, differentials, diagnosis, treatment, and prognosis for dogs with insulinomas.