Multiple hormone elevations in Zollinger-Ellison syndrome. Prospective study of clinical significance and of the development of a second symptomatic pancreatic endocrine tumor syndrome. In the present study of 45 patients with Zollinger-Ellison syndrome, the frequency and clinical importance of the release of multiple gastrointestinal peptides were assessed prospectively. During an initial evaluation, extent of gastrinoma, clinical symptoms, disease duration, and presence or absence of multiple endocrine neoplasia, type I (MEN-I) were assessed. All patients had determinations of fasting plasma gastrin, human pancreatic polypeptide, motilin, neurotensin, and somatostatin; 35 had determinations of insulin and gastrin-releasing peptide and 21 had determinations of glucagon. A plasma elevation of additional peptides besides gastrin was detected in 62%, with 44% having one, 18% having two, and 0% having three additional peptides elevated. Motilin was elevated in 29%, human pancreatic polypeptide in 27%, neurotensin in 20%, and gastrin-releasing peptide in 10%, whereas insulin, glucagon, and somatostatin were not elevated in any patient. The presence or absence of elevation of any peptide did not differ in patients with or without MEN-I, with gastrinoma size, with the presence or absence of metastatic disease, or with various clinical symptoms. Patients were assessed yearly for clinical evidence of a secondary symptomatic pancreatic endocrine tumor syndrome with a median follow-up of 146 and 84 months from onset or diagnosis, respectively. Only one patient (2% of patients) developed a second syndrome (rate, 2 patients per 100 patients observed for 10 years). These results demonstrate that the plasma elevation of multiple gastrointestinal peptides is common in patients with Zollinger-Ellison syndrome; however, the rate of developing a second symptomatic pancreatic endocrine tumor syndrome is much lower than generally believed. Furthermore, no evidence is found to support the conclusions that the detection of the plasma elevation of these peptides is clinically important in assessing MEN-I status, disease extent, or presence of metastatic disease or that elevated levels of motilin, neurotensin, gastrin-releasing peptide, or human pancreatic peptide are associated with any distinct clinical symptoms. Therefore, we recommend that plasma concentrations of these additional gastrointestinal peptides should not be assessed routinely but rather only if new symptoms develop. 