We describe an unusual case of extreme hepatic left lobar atrophy with hilar cholangiocarcinoma. A 67-year-old woman was referred to Nippon Medical School with obstructive jaundice. On admission, computed tomography revealed dilated intrahepatic bile ducts and a defect in the area drained by the left side of the middle hepatic vein. A Spiegel lobe was demonstrated, but the left lobe could not be detected to the left side of the gallbladder. Percutaneous transhepatic cholangiography was performed and demonstrated obstruction of the intrahepatic bile duct at the hepatic hilum. A drainage catheter was left in place. Angiography revealed that the left hepatic artery was present, but there was narrowing of the left portal vein. A diagnosis of agenesis of the left hepatic lobe with hilar cholangiocarcinoma was made. At surgery, the left lobe appeared extremely atrophic without atrophy of the Spiegel lobe. The right anterior branches of the hepatic artery and portal vein had been invaded by carcinoma, so a left trisegmentectomy was performed. Final pathology was advanced hilar cholangiocarcinoma with invasion of the hepatic parenchyma, portal vein, and nervous system. The left lobe was atrophic without hepatolithiasis. The left portal vein was narrow distal to the Spiegel branch. The serum total bilirubin concentration was elevated postoperatively, and the patient was treated for hepatic failure. The patient died of pneumonia without recurrence 7 months after surgery. This rare case of extreme hepatic left lobar atrophy with hilar cholangiocarcinoma was successfully treated by left trisegmentectomy. Preoperative portal embolization was unnecessary because the left lobe was already atrophic.