In the Cape Town Children's Hospital ascariasis is the commonest cause for an acute abdominal emergency; over a thousand cases have been admitted with ascariasis to the paediatric surgical wards. There was a high incidence of biliary ascariasis (424 cases) and routine intravenous cholangiography should be performed in all children with abdominal pain where ascariasis is suspected. The normal host/parasite relationship is described and the frequent invasion of the ampulla of Vater by the worm is discussed. The typical worm biliary colic is described and the classical surgical findings reported. The radiographic, ultrasonic and duodenoscopic diagnoses of the disease are evaluated. The management of the patient is described. Ninety-five percent of them were uncomplicated cases, but in 20 or 5% of the patients complications arose, the most important of them being ascariasis cholangitis, pyogenic cholangitis, perforation of the bile duct, cholecystitis and pancreatitis. The diagnosis and surgical management of these complications are described in some detail.