Immunohistochemical, histochemical, and morphometrical studies of six gastric carcinoid tumors and their possible precursor lesions in gastric mucosa are reported. All tumors, presenting successively at our institute, occurred in the corpus mucosa of 45- to 78-year-old patients. Two neoplasms had already metastasized. Three tumors contained gastrin- and/or serotonin-positive cells. Two groups of carcinoids, one with atrophic gastritis type A [AGA (four cases)], and one without AGA (two cases) were discerned. Only cases with AGA showed antral G cell hyperplasia consistently as well as fundic endocrine cell proliferation and sometimes multifocal tumors. This confirms previous reports that hypergastrinemia might be a predisposing condition for the development of gastric carcinoids in AGA. Fundic endocrine cell increase, verified by quantitative methods, was either diffuse or nodular. Diffuse endocrine cell hyperplasia comprised G, EC, and ECl cells. The observation that endocrine cell nodules comprised a similar mixture of endocrine cells, sometimes communicating with glands of pseudopyloric metaplasia and proving to be reversible in one case, provides evidence that these nodules are hyperplastic, but finally may lead to gastric carcinoid tumors in AGA. Size less than 150 micron, basal location, and mixed hormone content may be helpful criteria for the distinction of hyperplastic endocrine cell nodules from small carcinoid tumors.