Diffuse large B-cell lymphoma in the Revised European-American Lymphoma Classification encompasses various morphologic subtypes of diffuse large-cell lymphomas of B-cell origin. The category is biologically and clinically heterogeneous, even though it constitutes approximately 30% of all non-Hodgkin's lymphomas. Clinically, the International Prognostic Index that identifies high-risk group in aggressive non-Hodgkin's lymphomas is widely accepted. Lacking, however, are biologic or molecular prognostic markers that might aid in understanding the pathogenesis and designing specific therapies. CD44 isoforms are involved in tumor dissemination and might be associated with aggressive behavior of non-Hodgkin's lymphomas. We studied immunohistochemical expression of CD44s and CD44v6 in the tumors and examined their clinical significance in a cohort of patients with primary nodal diffuse large B-cell lymphoma who were uniformly evaluated and treated with doxorubicin-containing chemotherapy (n = 42). In contrast to CD44s signals, CD44v6 signals were weak in routinely processed non-Hodgkin's lymphoma sections. Therefore, we used a highly sensitive catalyzed reporter deposition system and successfully detected CD44v6 signals in diffuse large B-cell lymphomas. Overexpression of the isoform was verified by Southern blot of reverse transcription polymerase chain reaction products. CD44s and CD44v6 were positive in 17 (40%) of 42 and 13 (31%) of 42, respectively. CD44v6 was detected predominantly in lymphoma cells, whereas CD44s was often positive for nonneoplastic small lymphocytes as well. In univariate regression analysis, the B symptoms, being in the International Prognostic Index high-risk group, and CD44v6 expression emerged as significant parameters for poorer overall survival, but CD44s expression did not achieve statistical significance. When multivariate regression analysis was performed using the former three parameters, only CD44v6 expression remained significant (P = .017; relative risk = 3.48), indicating that CD44v6 is a molecule particularly important for predicting worse prognosis. CD44v6, which can be detected in the archival materials, might be a biologically and clinically useful marker in identifying the high-risk group in the diffuse large B-cell lymphoma category of the Revised European-American Lymphoma Classification.