Prospective screening was carried out in 12 members of three families with multiple endocrine adenopathies, type I (MEA,I) and in 14 patients with no multiple endocrine adenopathies with and without other endorcinopathies. Elevated basal and responsive (after a meal) plasma concentrations of a relatively new candidate-hormone, human pancreatic polypeptide (hPP), were associated with pancreatic apudoma tumors in three asymptomatic patients with multiple endocrine adenopathies, type I. Two of these patients had excision of the tumors that resulted in normal plasma hPP concentrations postoperatively. Both tumors contained hPP predominantly by immunocytochemistry; one, a pure pancreatic polypeptide apudoma, was studied extensively demonstrating also by radioimmunoassay a high content of hPP and negligible amounts of insulin, glucagon, somatostatin, vasoactive intestinal polypeptide and gastrin. In this patient plasma concentrations of other polypeptides including insulin, glucagon, somatostatin, vasoactive intestinal polypeptide, gastrin, parathyrin, thyrocalcitonin, prolactin, corticotropin, growth hormone, thyrtropin and amine, serotonin, were within normal limits. The other patient, after excision of an hPP-detected pancreatic mixed hPP-gastrinoma, also became eugastrinemic postoperatively. Normal basal plasma hPP concentrations, but with exaggerated hPP responses to a meal in 11 patients, were associated with various combinations of islet cell hyperplasia, antral G cell hyperplasia with moderate hypergastrinemia and parathyroid hyperplasia. The patients with multiple endocrine adenopathies who have demonstrated this type of increased hPP response to a meal have not been operated on but are at risk for islet hyperplasia. Four of the 12 patients with multiple endocrine adenopathies, type I, with both normal basal and normally responsive hPP concentrations have no evidence as yet of pancreatic involvement.