Purpose : To demonstrate CT findings of malignancies occurring in choledochal cysts focusing on thedifferential points with benign inflammation. Materials and Methods : The CT findings of seven patients withmalignancies occurring in choledochal cysts(six cholangiocarcinomas and one cholangiohepatoma) and three withbenign inflammatory wall thickening were reviewed. Six were studied with two-phase(arterial and portal) CT(threethe malignancies and all benign inflammations) and the remainder with conventional CT in the late portal orequilibrium phase. Spiral dynamic CT scans were performed in all two phase CT, except in the case of onemalignancy. The study was focused on the shape and enhancement pattern of the lesions and the presence of localinvasion or distant metastasis. Results : Three of seven associated malignancies showed concentric wallthickening(mean wall thickness=11.3mm), two eccentric, wall thickening and two polypoid masses. Two of threearterial phase CT scans showed tumor enhancement and one showed low attenuating concentric wall thickening, welldelineated from a strongly enhanced pancreas. In portal or delayed phase CT scans, all masses were isodense orslightly hypodense compared with the liver or pancreas. Extensive regional lymphadenopathy or distant metastasiswas present in six patients at the time of diagnosis. Three cases of benign inflammatory wall thickening showed athinner wall(mean thickness=5mm), and two showed arterial enhancement of the inner wall with only, a diffuse andeven pattern. On preoperative CT diagnosis, two cases of benign inflammatory wall thickening were misdiagnosed asmalignancies. Conclusion : Concentric wall thickening type was the most difficult to differentiate from benignwall thickening. Irregular wall thickening of more than 10mm and enhancement of the whole thickeness of the wallis a reliable sign of malignancy. Dynamic spiral CT is essential for this evaluation.