500 patients with hepatobiliary and pancreatic disease due to Ascaris lumbricoides infection were studied. 274 patients had duodenal ascariasis, 171 biliary ascariasis, 40 hepatic ascariasis, 8 gall bladder ascariasis, and 7 pancreatic ascariasis. Five clinical presentations were recognised: acute cholecystitis (64 patients), acute cholangitis (121), biliary colic (280), acute pancreatitis (31), and hepatic abscess (4). Ascarides in the duodenum (which were seen to invade only the ampullary orifice) induced either severe biliary colic or episodes of acute pancreatitis. 27 patients had pyogenic cholangitis and were managed by surgical (2) or endoscopic (25) biliary decompression and drainage. Removal of worms from the ampullary orifice and their extraction by mouth led to rapid relief of biliary colic (214 patients) and acute pancreatitis (16). 4 patients died (acute pancreatitis 2, pyogenic cholangitis 1, hepatic abscess 1). In 12 patients worms persisted in the biliary tree at 3 weeks; dead worms were removed from the biliary tree by surgery (5 patients) or with an endoscopic basket (7). Worms moved out of the ductal system in 211 patients. During a mean follow-up of 48 months (SD 14), 76 patients had worm re-invasion of the biliary tree due to ascaris re-infection. Intrahepatic duct and bile duct calculi developed in 7 patients in whom dead worms formed the nidus of stones.