This clinical report describes an 8-year-old Thoroughbred mare that was presented for evaluation of a chronic, unilateral nasal discharge. Findings on physical examination, radiology, and video-endoscopy supported a clinical diagnosis of ethmoidal hematoma. After surgical ablation of the mass a defect was detected in the cribriform plate. At necropsy a 1.5 cm aperture was identified in the left cribriform plate allowing direct communication between the fundus of the nasal cavity and the cranium. Histology of the mass identified tissue consistent with an adenocarcinoma. History of profuse epistaxis warrants further investigation to differentiate malignant lesions from the more common benign lesions. During surgical ablation of large, space-occupying masses of the caudodorsal nasal cavity the cribriform plate should be examined for defects or secondary erosive lesions.