Over a 12-month period (September 1984 to September1985), 64 children with newly diagnosed brain tumors were admitted to the Neurosurgical Service at The Children's Hospital of Philadelphia. Of these children, 29 had posterior fossa tumors. Of this population of children with posterior fossa tumors, three patients aged 4 months, 22 months, and 4 years old developed massive exsanguinating upper gastrointestinal hemorrhage within seven days of their primary neurosurgical procedure. In each instance, large posterior duodenal ulcers were encountered and were treated with oversewing of the duodenal ulcer and vagotomy-pyloroplasty. Follow-up currently ranges from 18 to 26 months. All three children have survived and none have had any gastrointestinal bleeding since then. Massive exsanguinating hemorrhage was not seen in children with brain tumors in locations other than the posterior fossa. In this population of patients, we advocate the use of prophylactic cimetidine and titration of gastric acidity with antacids.