An unusual urachal lesion, which is a mucinous adenocarcinoma arising in a giant urachal cyst and is associated with pseudomyxoma peritonei and stromal osseous metaplasia of the cyst wall, was examined in a 45-year-old male. The cyst was encapsulated, measured 22 x 20 x 20 cm and weighed 3800 g. The unilocular cavity was filled with mucin. Most of the cystic cavity was lined with tall, simple or stratified columnar epithelium with a focus of papillary projection into the cavity. These findings suggest that this cystic lesion represents cystadenoma rather than a simple cyst. Foci of invasive moderately differentiated mucinous adenocarcinoma were detected in the area of macroscopic papillary fronds. Carcinoembryonic antigen and CA19-9 were immunohistochemically positive for tumor cells and their serum levels were also elevated. Stromal dystrophic calcification was extensively observed in the cyst wall with foci of osseous metaplasia. Mucinous implants, which histologically demonstrated adherent mucinous masses without epithelial components on the surface, were observed in the pelvic and abdominal cavity, indicating pseudomyxoma peritonei.