Six cases of type A gastritis associated with multiple carcinoids and/or endocrine cell micronests (ECM) in the atrophic fundic mucosa were examined light microscopically, immunohistochemically, and ultrastructurally. The ECM and carcinoids were mainly composed of enterochromaffin-like (ECL) cells. The cells were hyperplastic only in the atrophic fundic glands and pseudopyloric glands, but not in the intestinal metaplastic gland. It is suggested that the development of both the ECM and the carcinoids is highly related to the atrophic change of the fundic mucosa and a trophic action of subsequently raised serum gastrin in type A gastritis and that the both lesions arise from the pseudopyloric glands or atrophic fundic glands. In addition, the definition of neoplastic ECM (microcarcinoid) of the stomach was made with comparative study on both the cases with ECM and multiple carcinoids and the cases with ECM alone.