Zollinger-Ellison syndrome is caused by tumors usually found in the head of the pancreas and the upper small bowel. These tumors produce the hormone gastrin and are called gastrinomas. High levels of gastrin cause overproduction of stomach acid.

High acid levels lead to multiple ulcers in the stomach and small bowel. Patients with Zollinger-Ellison syndrome may experience abdominal pain and diarrhea. The diagnosis is also suspected in patients without symptoms who have severe ulceration of the stomach and small bowel.

Gastrinomas may occur as single tumors or as multiple, small tumors. About one-half to two-thirds of single gastrinomas are malignant tumors that most commonly spread to the liver and lymph nodes near the pancreas and small bowel. Nearly 25% of patients with gastrinomas have multiple tumors as part of a condition called multiple endocrine neoplasia type I (MEN I). MEN I patients have tumors in their pituitary gland and parathyroid glands in addition to tumors of the pancreas.