A 17- year old Thoroughbred gelding used for high-level 3-day eventing was presented to the Cornell University Equine Hospital for surgical removal of a presumed progressive ethmoid hematoma. The gelding had a several month history of mild, intermittent, spontaneous epistaxis from his right nostril following exercise. The referring veterinarian diagnosed the gelding with a progressive ethmoid hematoma. The diagnosis was based on presence of sanguineous discharge from the right nasomaxillary opening on nasal endoscopy and presence of a well-defined soft tissue mass in the right caudal maxillary sinus in the area of the ethmoidal labyrinth on lateral and dorsoventral skull radiographs. A frontonasal sinus flap was performed under standing sedation with local nerve blocks, which revealed a large paranasal sinus cyst in the right caudal maxillary sinus. The surgical diagnosis was later confirmed by histopathology of the excised tissue.