A 49-year-old Japanese woman was found to have innumerable, small sessile elevations throughout the stomach as revealed by radiographic and endoscopic examination. Endoscopic biopsy showed that the elevations were formed by a diffuse proliferation of histiocytes in the lamina propria mucosae. The histiocytes were characterized by abundant interdigitating cytoplasmic projections, Birbeck granules, an oval or indented nucleus, and an absence of phagocytosis. They stained for S-100 protein but not for lysozyme, alpha-1-antitrypsin, or nonspecific antigen cross-reacting with carcinoembryonic antigen. Five and a half years after the first diagnosis, most of the elevated lesions disappeared, but a few histiocytoid cells were noted microscopically. Systemic examination showed no abnormalities outside the stomach. The patient has continued to be well. This case was diagnosed as primary benign histiocytosis X of the stomach. Four similar cases have been reported. This is the first case that was diagnosed using biopsy techniques and in which the natural history--i.e., from multiple polypoid extension throughout the stomach to spontaneous remission--was observed.