A 10-year-old male castrated Shetland sheepdog presented with a 24-hour history of vomiting and anorexia. Abdominal ultrasound disclosed a gallbladder distended with non-gravity dependent hyperechoic material containing stellate striations extending towards the gallbladder wall, which is diagnostic for a gallbladder mucocele. A gallbladder mucocele is defined as a formed, semi-solid to immobile, green-black mucoid mass in the gallbladder fundus. Gallbladder mucoceles are preferentially treated surgically as (1) histologic evidence suggests that the gallbladder itself is diseased, (2) formed mucoceles are unlikely to pass with choleretics alone, and (3) there is a risk of rupture and subsequent septic peritonitis. The patient was stabilized and a cholecystectomy performed. He recovered uneventfully and was subsequently diagnosed and treated for hyperadrenocorticism, one of the endocrinopathies associated with the development of gallbladder mucoceles. Endocrinopathies are hypothesized to contribute to gallbladder dysmotility, mucosal cystic hyperplasia of the gallbladder wall, and altered bile composition.