A gross and microscopic study has been made of the changes associated with aging in the distal common bile duct of normal humans. Grossly a progressive narrowing develops with aging at the level of the preampullary portion of the common bile duct. On the contrary, the pancreatic portion of the common bile duct has a tendency to dilatation with aging. Histologically the thickening and sclerosis of the wall in the preampullary portion represent the changes increasing in degree with aging, consisting of: 1) submucosal glandular proliferation and fibrosis, and 2) intermuscular glandular proliferation and fibrosis. On the other hand, the diffuse sclerosis in the pancreatic portion gradually occurs with aging, consisting of: 1) proliferation and fragmentation of elastic fibers, and 2) increase in collagen fibers. Factors influencing the development of these changes associated with age are discussed. The author presumes that decrease in response to an upward tendency of the intraductal pressure resulting from diffuse sclerosis in the wall of the common bile duct, in addition to a progressive narrowing of the preampullary portion, may be responsible for dilatation of the common bile duct, moreover, bile stasis and infection on the biliary tract. It is suggested therefore that these alterations with aging process in the terminal bile duct may be a responsible agent in the formation of common duct stones.