Progressive ethmoid hematomas are uncommon lesions of unknown etiology that affect more commonly horses 6 years of age or older. Endoscopic examination commonly reveals a characteristic gray to white mass in the region of the ethmoids. The most common clinical sign is mild, intermittent, unilateral epistaxis caused by ulceration of the respiratory epithelium covering the mass or by destruction of adjacent tissues. Several treatment options are available to attempt mass removal. Regardless of treatment, prognosis for recurrence of ethmoid hematomas is high.