PURPOSE: To demonstrate CT findings of malignancies occurring in choledochal cysts focusing on the differential points with benign inflammation. MATERIALS AND METHODS: The CT findings of seven patients with malignancies occurring in choledochal cysts(six cholangiocarcinomas and one cholangiohepatoma) and three with benign 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 or equilibrium 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 wall thickening(mean wall thickness=11.3mm), two eccentric, wall thickening and two polypoid masses. Two of three arterial phase CT scans showed tumor enhancement and one showed low attenuating concentric wall thickening, well delineated 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 metastasis was 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 and even pattern. On preoperative CT diagnosis, two cases of benign inflammatory wall thickening were misdiagnosed as malignancies. CONCLUSION: Concentric wall thickening type was the most difficult to differentiate from benign wall thickening. Irregular wall thickening of more than 10mm and enhancement of the whole thickeness of the wall is a reliable sign of malignancy. Dynamic spiral CT is essential for this evaluation.