We prospectively evaluated the incidence, clinical features, radiographic findings, and course of biliary and pancreatic disease caused by ascariasis in an endemic area in India. Ascariasis was an etiologic factor in 40 (36.7%) of the 109 patients studied who had biliary and pancreatic diseases. Disease was prevalent in adult women and was associated with recurrent biliary colic in 38 patients (95%), recurrent pyogenic cholangitis in 27 patients (68%), acalculous cholecystitis in 9 patients (23%), and pancreatic disease in 6 patients (15%). Vomiting of roundworms during biliary colic occurred in 19 patients (48%) and often led to confirmation of biliary ascariasis by direct visualization of the biliary tree. Endoscopic retrograde cholangiopancreatography was an excellent diagnostic tool and often demonstrated worms in the dilated common bile duct and intrahepatic ducts. The worms moved actively into and out of the biliary tree from the duodenum. Thirty-six (90%) patients recovered on symptomatic treatment followed by anthelmintic therapy once acute symptoms subsided. Surgery was needed in 4 patients, as the worms were trapped in the ducts and had led to the formation of common bile duct and intrahepatic duct stones with the worm fragment as the nidus.