We describe a cystic intraductal papillary neoplasm of bile duct (IPNB) of adenocarcinoma treated with surgical resection. An 82-year-old man was admitted to our hospital because of distension of the right upper quadrant of the abdomen. On admission, a huge nontender mass was palpated in the right upper abdomen. Initial laboratory tests revealed the following serum values: aspartate aminotransferase, 123 IU/L; alanine aminotransferase, 113 IU/L; alkaline phosphatase, 376 IU/L; bilirubin, 1.6 mg/dL; alpha-fetoprotein, 7.4 ng/mL; CA19-9, 39.8 U/mL; carcinoembryonic antigen, 4.1 ng/mL; and Duke pancreatic monoclonal antigen type 2, 200 U/mL. Abdominal ultrasonography revealed an anechoic mass with an elevated papillary lesion in the right paramedian section. Computed tomography showed a low-density hepatic lesion, measuring 12 × 10 cm, with thick, irregular walls. A T2-weighted magnetic resonance scan showed mural nodules with signal intensity higher than that of the liver. The cystic lesion was suspected to be a cystic IPNB, but we could not determine whether it was a carcinoma. We performed extensive right paramedian sectionectomy and cholecystectomy. The resected multilocular tumor was filled with a yellow fluid, and the cut surface showed multiple, yellowish papillary nodules lining the cystic wall. On histologic examination, the neoplastic biliary epithelium showed papillary growth in the dilated lumen. Papillary and micropapillary lesions exhibited cellular atypia: nuclear enlargement and hyperchromasia, mutilayering, and mitosis. Ovarian-like stroma was not detected. The tumor was diagnosed as an IPNB (carcinoma in situ lesion). These lesions had expanded into several intrabiliary ducts. No recurrence has been detected 2 year after operation.