We previously reported that the prognosis of CD21-positive diffuse large B-cell lymphoma (DLBCL) is significantly favorable to that of CD21-negative DLBCL (Otsuka et al. in Br J Haematol 127:416-424, 2004). In this study, we attempted to clarify the biological significance of CD21 expression in B-cell lymphoma (BCL) by performing in vitro experiments using CD21 transfection into a CD21-negative lymphoma cell line and analyzing clinical data from lymphoma samples. Established clones of CD21 transfectants showed homotypic aggregation in suspension culture. Analysis of integrin expression revealed that LFA-1 appeared to be expressed on CD21 transfectants, and the cell aggregation was abrogated by anti-LFA-1 antibody. The CD21 transfectants could adhere to plastic plates coated with ICAM-1. Moreover, flow cytometry and/or immunohistochemical analyses of clinical BCL samples (n = 29) revealed positive for CD21 in all cases; LFA-1 was also expressed without exception. All BCL cells isolated from cavity fluids (n = 10) failed to express both CD21 and LFA-1. These data suggest that CD21 is tightly related to LFA-1 expression in BCL and the absence of CD21/LFA-1 expression is associated with pleural/peritoneal fluid involvement by BCL, a potential indicator of disease progression of BCL.