A 20-year-old Warmblood broodmare was presented to her primary veterinarian for bilateral nasal discharge, which was treated with sulfamethoxazole and did not resolve although the discharge turned from white to clear. The mare was referred to Cornell’s Equine and Nemo Farm Animal Hospital (ENFAH) in January 2020 after there was no resolution of signs and she began dropping feed without losing weight. Oral exam, radiography, computed tomography (CT), and endoscopy were performed and revealed a left sided nasal abscess filled with feed material due to an associated oronasal fistula between left maxillary third and fourth premolars (207 and 208). The mare underwent standing surgery to establish drainage and debride the abscess. The mare continues to have monthly recheck upper respiratory endoscopies and debridement to improve quality of life, but the prognosis for periodontal health is guarded. This seminar highlights the common causes of nasal discharge in horses, the normal anatomy of the equine paranasal sinuses, and the possible origins of the abscess.