Clinical Scenario A man is referred to you for evaluation of possible Zollinger–Ellison syndrome (ZES).1 He gives an 8-year history of heartburn, for which he has been receiving increasing doses of proton pump inhibitor (PPI) therapy. Over the past few months he has noted worsening symptoms of heartburn and regurgitation, especially at night or after large meals. His primary care physician increased his maintenance PPI dose of omeprazole from 40 mg once daily to 40 mg twice daily but his symptoms have persisted and now he also is complaining of abdominal pain that wakens him from sleep. Open-access, upper-gastrointestinal endoscopy was unrevealing but his fasting serum gastrin level was 465 pg/mL (normal, 100 pg/mL). When this result was obtained his primary care physician ordered an Octreoscan (Mallinckrodt Inc., St Louis, MO) (which was normal) and a serum chromogranin A level (which was increased at 492 pg/mL [normal, 375 pg/mL]). The patient describes no recent change in bowel habits and there is no history of anemia, weight loss, bleeding, jaundice, or dysphagia. Physical examination is within normal limits.