A unique case of adenocarcinoma arising in a retroperitoneal bronchogenic c
yst is presented. A 55-year-old woman presented with lower abdominal discom
fort. Computed tomography revealed a retroperitoneal cystic mass attached t
o the ascending colon. The resected cyst was unilocular and filled with mil
ky white mucus and hemorrhagic debris. Histologically, most of the cyst wal
l was of well-differentiated papillary adenocarcinoma with no cyst wall inv
asion. Other small areas of the cyst were lined with variably atypical dysp
lastic/metaplastic cuboidal to pseudostratified columnar epithelium. The cy
st wall was mostly hyalinized, but there was apparent thickened subepitheli
al basement membrane, elastosis, and a single layer of smooth muscle that s
uggested bronchial wall structures. A mucin staining study with O-acylated
sialic acid, which is used for the demonstration of gastrointestinal, chole
cystic and uterine cervical mucins, was negative for the mucin-producing ep
ithelial cells of the cyst, Thus, to our knowledge, this is the first repor
ted case of adenocarcinoma arising in a retroperitoneal bronchogenic cyst.