A combination of a cholecystectomy and detachment of the supportive tissue surrounding the extrahepatic bile duct results in chronic bile duct dilation. To clarify whether this cholangiectasis is due to the detachment of the surrounding tissue or to the cholecystectomy, we have studied 4 groups of adult mongrel dogs, i.e., a control group given a sham operation (n = 20), a group given cholecystectomy (n = 20), a group that had the surrounding tissue detached from the bile duct (n = 20), and a group given both a cholecystectomy and detachment of the surrounding tissue (n = 29). On examination by cholangiography at 1, 2, 3, and 4 weeks after the initial laparotomy, no significant cholangiectasis was found in dogs subjected to either cholecystectomy alone or to detachment of the surrounding tissue alone. In contrast, in dogs subjected to a cholecystectomy combined with detachment of the bile duct, the bile duct gradually dilated, resulting in significant cholangiectasis at 4 weeks. However, the intrabiliary pressure, the residual pressure, and the actual resistance value, which are indices of a passage disturbance in the sphincter of Oddi, were normal in all 4 groups. It is therefore concluded that both the extirpation of the gallbladder, and the lack of supportive tissue surrounding the bile duct were responsible for the resulting cholangiectasis.