Macroscopic examination of the extrahepatic bile ducts in infants with biliary atresia reveals fibrous occlusion of variable extent from an inflammatory process of unknown aetiology. Histological studies have shown that bile duct remnants at the porta hepatis frequently contain small epithelium-lined channels which communicate with intrahepatic ducts and through which effective bile drainage may be established by the operation of portoenterostomy. The 4-year survival rate in untreated cases is 2 per cent, but surgical treatment can improve the outlook and recent reports suggest that a 5-year survival rate of over 35 per cent can be achieved with portoenterostomies performed before 10 weeks of age. Complications after surgery include progressive liver disease, ascending bacterial cholangitis and portal hypertension.