Functioning tumors of the pancreatic islets are now recognized as the source of clinical syndromes affecting the gastrointestinal tract which have a wide variety of catastrophic symptoms. Experiences with thirty-six cases suggest at least four separate diagnostic categories in the ulcerogenic tumor syndrome. These include: a typical history, gastric analysis, and roentgenographic findings with boderline fasting serum gastrin levels; ulcerogenic tumor with evidence of hyperparathyroidism; iatrogenic ulcerogenic syndrome associated with failure of a previous operation for duodenal ulcer; and the classic ulcerogenic syndrome associated with a fulminating ulcer diathesis or diarrhea and high serum gastrin levels. The problems presented at operation include: decisions to be make in the presence of a negative exploration; the finding of a solitary tumor in the wall of the duodenum; solitary pancreatic tumors particularly in the body and tail; ulcerogenic tumors in the very young; liver metastases in the elderly; and the wisdom of removing gross metastases in combination with total gastrectomy. The long-term survival in the ulcerogenic tumor syndrome approximated 50 per cent, with 40 per cent of those having proved malignancy living five years. Evidence of hyperparathyroidism is relatively common in association with both the ulcerogenic and the diarrheogenic tumor syndromes. The association may by a result of a congenital abnormality, metabolic alkalosis, or a direct effect of the islet cell tumor. Parathyroidectomy may be indicated when both the serum calcium and parathormone levels are elevated in the presence of borderline fasting gastrin levels. The latter may return to normal after parathyroidectomy. The evidence of hyperparathyroidism closely parallels the episodes of diarrhea in the diarrheogenic syndrome, and hyperparathyroidism may regress spontaneously after total removal of the pancreatic tumor. Just as routine calcium determinations made the diagnosis of hyperparathyroidism more commonplace, it is suggested that the gastrointestinal syndromes associated with islet cell tumor would receive wider recognition if radioimmunoassays for gastrin as well as secretin, and the other secretin-like polypeptides, were carried out routinely.