AbstractIt can be difficult to distinguish among poorly differentiated carcinomas from large cell malignant lymphomas of the alimentary tract or malignancies of true histiocytes (tumor of monocyte/macrophage lineage) (1). Immunoperoxidase methods, once investigative tools, are now commonly used to solve these diagnostic problems as there is now an appreciation that the biologic nature of a tumor can be determined from its cellular and surface constituents (2). Because planning and evaluation of therapy requires knowledge of the cell type of a particular tumor, immunological studies are often crucial in accurate diagnosis. This is a report of a true histiocytic tumor of the gastro-esophageal junction, the diagnosis of which required immunological characterization.