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Twenty-eight gastric wall tumors classified by light microscopy as leiomyomas or leiomyosarcomas were reevaluated for histogenesis. Each case was analyzed for the presence of S-100 protein, a marker for cells derived from neuroectoderm, by the immunoperoxidase technique. Eight tumors contained cells with positive immunostaining for S-100 protein. Usually this staining was focal, but in one case staining was diffuse. In three additional cases the immunostaining outlined a nerve within the tumor. In contrast, two esophageal and 10 uterine leiomyomas, as well as normal gastric smooth muscle, gave negative reactions for S-100 protein. Twelve cases had tissue processed for electron microscopy. Only two of the tumors, one leiomyoma and one leiomyosar-coma, contained cytoplasmic myofilaments with densities expected in cells derived from smooth muscle; neither of these tumors stained for S-100 protein. In one case, the tumor with diffuse staining for S-100 protein, the cells resembled Schwann cells ultrastructurally. The remaining nine cases had neither smooth muscle nor Schwann cell features. They did contain interposed cell processes, primitive junctions, and large cytoplasmic vacuoles. The results of this study indicate that many gastric wall tumors are not derived from smooth muscle. The presence of S-100 protein suggests a nerve sheath origin in some cases. While the ultrastructure of many gastric tumors does not resemble nerve sheath cells in most peripheral nerves, the myenteric nervous system is a possible source for perineurial or mesenchymal nerve sheath cells with distinctive fine structure. Further study is needed to refine our knowledge of the histogenesis of gastric stromal tumors.