Advances in the molecular definition of surface proteins (adhesion molecules) involved in tumor metastasis may help to explain the invasive behavior of malignant tumors, that is, the migration of tumor cells involving reversible adhesive contacts, their release in the circulation, and their extravasation into distant sites. Intercellular adhesion molecule-3 (ICAM-3), the third receptor for the lymphocyte function-associated antigen molecule-1 (LFA-1) was recently characterized. We investigated fresh frozen skin biopsies from 10 patients with mycosis fungoides, four with pleomorphic T-cell lymphoma, six with Sezary syndrome, 10 with primary cutaneous B-cell lymphoma, and 10 with eczematous lesions as controls. The biopsies were compared with lymph node biopsies of five patients with known cutaneous T-cell lymphoma (CTCL), 10 with primary nodal B-cell lymphoma, and 11 with lymph-node specimens showing dermatopathic lymphadenopathy as controls. The specimens were stained with ICAM-3 antibody (Bender Medical Science) using the alkaline phosphatase antialkaline phosphatase method. Using cytomorphologic criteria, neoplastic lymphocytes could be differentiated from smaller reactive cells. Staining intensities were classified semiquantitatively as follows: 4, strong expression in 75 to 100% of the tumor cells; 3, 50 to 75%; 2, 25 to 50%; 1, 5 to 25%; and 0 fewer than 5% of the tumor cells. The endothelial cells in skin biopsies of seven of 30 primary cutaneous lymphomas expressed ICAM-3. In contrast, no expression of ICAM-3 could be demonstrated on endothelial cells in lymph nodes infiltrated with tumor cells of CTCL. Finally, endothelial cells of lymph nodes infiltrated with primary nodal B-cell lymphomas showed expression of ICAM-3 in three of 10 patients. The endothelial cells in the 11 control patients presenting with both eczematous lesions and dermatopathic lymphadenopathy showed no staining for ICAM-3. Every patient who expressed ICAM-3 on endothelial cells showed systemic spread of this disease. The findings suggest that ICAM-3 expression may be induced on endothelial cells in late-stage cutaneous lymphomas, probably by a cytokine-mediated mechanism.