A mammary gland neoplasm was identified at necropsy in a 9-year-old ovariectomized Holstein cow with a 1-year history of an enlarged left hindquarter. A sterile serous secretion was expressed from the quarter. The cow developed Corynebacterium pyogenes infection in the quarter one year later and died of secondary complications. The left hindquarter was found to contain multiple cysts. The histologic diagnosis was multicentric papillary cystadenoma.