A unique case of adenocarcinoma arising in a retroperitoneal bronchogenic cyst is presented. A 55-year-old woman presented with lower abdominal discomfort. Computed tomography revealed a retroperitoneal cystic mass attached to the ascending colon. The resected cyst was unilocular and filled with milky white mucus and hemorrhagic debris. Histologically, most of the cyst wall was of well-differentiated papillary adenocarcinoma with no cyst wall invasion. Other small areas of the cyst were lined with variably atypical dysplastic/metaplastic cuboidal to pseudostratified columnar epithelium. The cyst wall was mostly hyalinized, but there was apparent thickened subepithelial basement membrane, elastosis, and a single layer of smooth muscle that suggested bronchial wall structures. A mucin staining study with O-acylated sialic acid, which is used for the demonstration of gastrointestinal, cholecystic and uterine cervical mucins, was negative for the mucin-producing epithelial cells of the cyst. Thus, to our knowledge, this is the first reported case of adenocarcinoma arising in a retroperitoneal bronchogenic cyst.