Gastric ulceration developed in eight patients during intrahpeatic arterial infusion of 5-FU. Bleeding occurred in four instances and perforation in one. In all cases the catheter tip had been dislodged and was proximal to its correct position, allowing the stomach to be directly infused with 5-FU. No duodenal ulcers were noted. All patients were symptomatic for several days before the diagnosis was made. Of 20 patients with catheter dislodgement, five had documented ulcers, three had upper gastrointestinal bleeding of undetermined etiology, eight had epigastric pain or vomiting and only four were asymptomatic. Prompt determination of catheter position is necessary in patients receiving intrahepatic arterial infusion of 5-FU if symptoms consistent with gastric ulceration occur. Gastric ulcers should be vigorously treated because of the high rate of complications in patients receiving chemotherapy.