BACKGROUND AND AIMS: Patients with a choledochal malformation, formerly described as cysts, are at increased risk of developing a cholangiocarcinoma and resection is recommended. Given the low incidence of CM in western countries, the incidence in these countries is unclear. Our aim was to assess the incidence of malignancy in CM patients and to assess postoperative outcome. METHODS: In a nationwide, retrospective study, all adult patients, who underwent surgery for CM between 1990 and 2016 were included. Patients were identified through the Dutch Pathology Registry and local patient records and were analysed to determine the incidence of malignancy, as well as postoperative mortality and morbidity. RESULTS: A total of 123 patients with a CM were included in the study (Todani Type I, n=71; Type II, n=10; Type III, n=3; Type IV, n=27; unknown, n=12). Median age was 40 years (range 18-70) and 81% were female. The majority of patients (99/123) underwent extrahepatic bile duct resection, with additional liver parenchyma resections in eight patients, only exploration in two, and a local cyst resection in eight patients. Postoperative 30-day mortality was 2% (2/123) and limited to patients who underwent liver resection. Severe morbidity occurred in 24%. In 14 of the 123 patients (11%) a malignancy was found in the resected specimen. One patient developed a periampullary malignancy 7 years later. CONCLUSIONS: In a large Western series of CM patients 11% were found to have a malignancy. This justifies resection in these patients, despite the risk of morbidity (24%) and mortality (2%).