The histologic boundaries of the extrahepatic bile ducts (EBDs) are not well defined, despite the fact that pathologic staging of carcinomas arising in these structures requires the determination of extent of tumor invasion in this area. Perhaps in part, because the smooth muscle band in the EBD wall is not well formed throughout the length of these structures, a previous definition of the outer portion of the bile duct wall included "loose, richly vascularized connective tissue, interlaced with large nerve fibers." We have experienced difficulty in the application of these criteria in staging EBD carcinomas, which requires the histologic determination of the extent of the EBD wall. To systematically study the histologic features of EBD tissue boundaries, 34 EBD specimens obtained from autopsy were analyzed with attention to the distribution of blood vessels and nerve fibers along the length of this system. The EBD specimens were divided into lower, middle, and upper portions, and the locations of blood vessels and nerve fibers were then analyzed separately at each location. We defined the fibromuscular wall as the dense concentric arrangement of collagen and smooth muscle fibers surrounding the EBD mucosa. The location of blood vessels and nerve fibers was categorized as either (1) within, (2) junctional to, or (3) outside of the fibromuscular wall. Blood vessels and nerve fibers are located predominantly outside of the fibromuscular wall and are usually surrounded by adipose tissue throughout the entire EBD, however, their distribution in this location is not consistent. Because of these histologic features, we propose that the bile duct wall is more precisely defined as occurring between the mucosa and the outermost boundary of dense fibromuscular tissue, without consideration of the presence or absence of blood vessels and nerve fibers.