A thirteen year old neutered female Domestic Shorthair cat with oral squamous cell carcinoma was noted to have a cranial mediastinal mass on survey thoracic radiographs. Fluoroscopic aspiration was performed and cystic fluid was obtained which was clear colorless, and devoid of cells, with a specific gravity of 1.005 and protein<2.5 mg/dl. On postmortem examination, a unilocular cyst was identified originating from the external lateral surface of the cranial mediastinum. Histologically, the cyst lining was nonciliated and resembled mesothelium. These findings differentiate this cyst from previously reported branchial cysts in cats where the cysts were multiloculated, the cyst fluid was mucinous and cellular, and the cysts were lined by ciliated columnar epithelial cells.
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