A renal tubular cystadenoma was diagnosed in a 14-yr-old male African lion (Panthera leo). During a routine health evaluation, a left renal mass was identified via physical examination, radiographs, and abdominal ultrasonography. The mass was 30 × 15 cm in size and had a thin capsule with central hypoechoic fluid, suggestive of a perirenal cyst. An exploratory celiotomy with partial nephrectomy was performed without complications. Histologically, the tumor was characterized by a thick fibrous capsule surrounding multiple, variable-sized cysts that markedly compressed the adjacent fibrotic and atrophied renal cortex. Immunohistochemical labeling for Aquaporin-1 and Tamm-Horsfall protein was consistent with a renal tubular cystadenoma of proximal tubule origin. Renal cystadenomas are an uncommon benign epithelial neoplasm. There are only two documented case reports in domestic cats. This report represents the first documentation, to the authors' knowledge, of a renal cystadenoma in a lion.
