A 25-year-old woman was referred to our hospital with persistent upper abdominal pain. Preoperative imaging studies revealed a hilar bile duct stricture with portal venous encasement, and the patient underwent curative resection involving extended left hepatectomy and segmental portal vein resection. The pathological findings demonstrated a well-differentiated tubular adenocarcinoma of the bile duct with regional lymph node metastasis (stage IIIB according to the UICC TNM classification), as well as the overexpression of p53 proteins and the K-ras gene mutation in tumor cells. The patient has shown no evidence of recurrence in the 10 months since the operation. Although there are several reports of relatively young adults with cholangiocarcinoma, the majority of such patients demonstrate either an anomalous arrangement of the pancreaticobiliary duct system or primary sclerosing cholangitis. The absence of any morphological abnormalities in this patient's biliary system implicates de novo carcinogenesis as the most likely cause of the cholangiocarcinoma.