Site - Duodenum> pancreas > intraabdominal sites (mesentery, lymph nodes, biliary tract, liver, stomach, Ovary) The diagnosis of ZES requires the demonstration of inappropriate fasting hypergastrinemia, usually by demonstrating hypergastrinemia occurring with an increased basal gastric acid output (BAO) (hyperchlorhydria). Chronic unexplained diarrhea also should suggest gastrinoma. Approximately 20-25% of patients with ZES have MEN 1, and in most cases hyperparathyroidism is present before the gastrinoma. These patients are treated differently from those without MEN 1; therefore, MEN 1 should be sought in all patients by family history and by measuring plasma ionized calcium and prolactin levels and plasma hormone levels (parathormone, growth hormone).