Multiple gastric carcinoids are a well-recognized complication of hypergastrinemia associated with chronic atrophic gastritis. However, the management of large tumors (>2 cm in diameter) remains uncertain, with the decision between antrectomy or total gastrectomy being empirical. This report describes the investigation of a patient with chronic atrophic gastritis and multiple large gastric carcinoid tumors. Before surgery, octreotide was infused for 72 hours to suppress enterochromaffin-like (ECL) cell and gastrin cell function. The infusion decreased plasma gastrin and gastrin synthesis; moreover, there were marked reductions in markers of ECL cell function, e.g., histidine decarboxylase and chromogranin A messenger RNA abundance, in carcinoid tumor tissue and macroscopically normal corpus mucosa. An antrectomy was performed, after which the patient made an uneventful recovery. Six months after surgery, a single residual polyp, enriched with smooth muscle cells but not ECL cells, was removed. One year after antrectomy, the remaining stomach was normal. The response of ECL cell markers in carcinoid tissue to octreotide suggested that these cells were under neuroendocrine control and, therefore, predicted a beneficial outcome for antrectomy. It is suggested that an octreotide supression test coupled with assay of histidine decarboxylase or chromogranin A gene expression is useful in the assessment of gastric carcinoid tumors.