Antrectomy has been proposed as the preferential treatment of hypergastrinemic patients with nonantral gastric carcinoids since it removes the main growth factor for the tumors and their precursor lesions, ie, hypergastrinemia. To investigate the cellular basis of the mechanism for postantrectomy regression in nonantral endocrine cells, a light and electron microscopic morphometric study was performed in a case of enterochromaffinlike-cell gastric carcinoid associated with hypergastrinemia before and 4 and 10 months after antrectomy. The withdrawal of sustained hypergastrinemia obtained by antrectomy was associated with a progressive reduction of the volume density, cross-sectional area, and number of profiles of endocrine cells in the remaining nonantral mucosa, in which gastrin-dependent proliferations were regarded as the carcinoid precursor lesions. Ultrastructural morphometry demonstrated that the changes selectively involved the enterochromaffinlike cells, ie, the specific cell target for the trophic action of gastrin and the usual component of gastric carcinoids. The volume fractions of enterochromaffin and X cells (producing serotonin and endothelin, respectively) were increased 10 months after antrectomy. Persistence of a modest elevation of gastrin levels, likely due to the occurrence of gastrin cells in areas of pyloric metaplasia of the nonantral mucosa, did not prevent the hypotrophic effects of antrectomy.