The purpose of this study is to describe the arterial supply of the entire extrahepatic bile duct system. The cross-sectional area of all arteries that supply the ducts is measured under an operating microscope in 50 adult cadavers injected with red latex through the aorta. The extrahepatic bile duct system is divided into four topographic portions: cystic duct and gallbladder, right and left hepatic ducts, bile (common) duct and including its supraretroduodenal parts, and the pancreatic and intraduodenal portions. The arterial supply to each portion is carefully detailed. The ducts are supplied by more than seven arteries, of which the major arteries are the cystic artery, posterior superior pancreaticoduodenal artery, right hepatic artery, and retroportal artery. Collectively they provide 94.5% of the blood supply to the ducts. Arteries form three types of anastomotic patterns on the walls of the ducts, suggesting that ductal incisions can be made in ways that least disturb the blood supply. The patterns are: a network, a longitudinal anastomotic chain, and an arterial circle. These data emphasize the importance of the arterial supply in biliary surgery and especially the treatment of hemobilia.