BACKGROUND: Caroli's disease is the dilatation of the segmental intrahepatic bile ducts. It usually affects the entire liver but can occasionally involve only one lobe, commonly the left. This study included 12 cases of unilobular disease, nine localised in the left lobe and three in the right lobe, which were all treated by liver resection. PATIENTS AND METHODS: These 12 patients underwent surgery between 1974 and 1997. There were six men and six women (mean age: 51 years). The initial presentation and diagnosis were reported. The mean interval between the first symptoms and diagnosis was 12.5 years. Eight of the 12 patients had undergone 22 surgical or endoscopic procedures prior to liver resection. In the present series a preoperative ultrasonogram or CT scan established the diagnosis in all cases. Six patients did not have stones in the gallbladder. RESULTS: Surgical treatment consisted in seven left lobectomies, two left hepatectomies and three right hepatectomies (Couinaud's classification). A intrahilar cholangiojejunostomy was performed in five cases. Pathological examination showed cystic dilatation of the intrahepatic segmental and subsegmental bile ducts, measuring from a few millimetres to 4 cm, which contained calculi. Two cases were associated with congenital hepatic fibrosis. An intrahepatic focus of ectopic pancreatic tissue was seen in one case. There were no cases with cholangiocarcinoma. One patient developed a biliary fistula which required reoperation. All patients had an uneventful long term postoperative course except for one patient who died of colon carcinoma 3 years postoperatively. CONCLUSION: When associated with other malformations, most notably congenital hepatic fibrosis, this commonly diffuse disease is called Caroli's syndrome. The unilobar form, most usually involving the left lobe of the liver, is called Caroli's disease. Both monolobar and diffuse types are often characterised by recurrent bouts of cholangitis and, in over half of the cases, by common bile duct stones without gallbladder stones