Lymphocyte function-associated antigen 1 (LFA-1) is a glycoprotein involved in virtually all aspects of the immune response requiring direct cell to cell contact. It has been suggested that lack of LFA-1 expression in lymphomas may represent a mechanism of escape from immunologic surveillance. We investigated the expression of LFA-1 in a series of more than 250 lymphoid neoplasms and reactive lymphoid proliferations using a frozen section immunoperoxidase technique. LFA-1 was expressed by all lymphoid populations in the reactive cases. In contrast, absence of LFA-1 alpha or beta chains was found in 44% of non-Hodgkin's lymphomas, including 50% of B-cell lymphomas. These findings suggest that loss of LFA-1 expression may be of great use in the differential diagnosis of benign versus malignant lymphoproliferations. Eighty percent of initial biopsy specimens of low-grade lymphoma exhibited LFA-1 expression, whereas only 8% of recurrent specimens retained expression of both LFA-1 subunits. However, we found no correlation between LFA-1 expression and clinical course in a series of 64 patients with diffuse large cell lymphomas.