Tumor-associated macrophages (TAM) are one of the factors which modulate the carcinoma progression. The present study described immunohistochemical expression of intercellular adhesion molecule-1 (ICAM-1) in stromal cells in human gastrointestinal carcinoma identifying the cell types by immunoelectron microscopy. In colon and gastric carcinomas, ICAM-1-positive cells were mostly stromal cells, and major cell types were identified as macrophages and fibroblasts by immunoelectron microscopy. Macrophages were characterized by their ovoid shape, cytoplasmic projections, abundant vacuoles, phagocytosis, and paucity of rough endoplasmic reticulum. Fibroblasts contained stacks of rough endoplasmic reticulum. Macrophages were major cells among ICAM-1-positive cells along the invasive margin, while fibroblasts were predominant in the stroma within carcinoma in colon and intestinal-type gastric carcinomas. Lymphocytes positive for lymphocyte function associated antigen (LFA-1), a counter-receptor of ICAM-1, were densely distributed along the invasive margin, and sparsely in the stroma within carcinoma. In diffuse-type gastric carcinoma, most macrophages were dendritic-shaped and negative for ICAM-1. Our study suggests that the invasive margin is an area similar to active inflammation, where the antigen presenting cells (macrophages) and lymphocytes may interact via the ICAM-1/LFA-1 adhesion.