A series of 127 surgical specimens of epithelioid leiomyomatous tumors (leiomyoblastomas) of the gastric wall from the files of the Armed Forces Institute of Pathology (AFIP) were studied as to biologic behavior, morphogenesis, and histologic features of value in distinguishing benign and malignant variants. These tumors affect middle-aged men primarily and usually present with upper gastrointestinal bleeding or peptic ulcer-like symptoms. They are composed of a mixture of round epithelioid and spindle cells, many of which have clear cytoplasm. The cells are ensheathed by delicate reticular fibers. The presence of a perithelial or glomoid pattern in some tumors suggests a possible relationship to angiomyoma, glomus tumors, and "pericytoma." The epithelioid leiomyoma, the benign form, often arises in the mid- and distal stomach, especially on the anterior wall. Microscopically, it is recognized by the presence of large epithelioid cells and infrequent mitotic figures. Of 103 epithelioid leiomyomas, only one metastasized and thus was biologically malignant. The epithelioid leiomyosarcoma often arises in the proximal stomach and also distally, especially on the posterior wall. Two histologic types of epithelioid leiomyosarcoma are distinguished from the benign epithelioid leiomyoma by the small size of the cells and occasional higher mitotic counts. One sarcoma variant is a small cell caricature of the leiomyoma. The other is more anaplastic, assoicated with a loss of reticular fibers surrounding the cells and an alveolar arrangement. Epithelioid leiomyosarcomas are the most common type of gastric sarcoma. They are aggressive neoplasms; 63% metastasized, usually within 2 years after diagnosis.