Primary Sjögren's syndrome (pSS) is an autoimmune disorder characterized by lymphocytic infiltration of the salivary glands. Most of the infiltrating cells are T cells, but other features of the disease include polyclonal B-cell activation, systemic production of autoantibodies, and increased risk of developing B-cell non-Hodgkin's lymphoma. Recently, a new tumour necrosis factor, the B-cell activating factor (BAFF; also known as BLyS), has been implicated in the polyclonal activation of B cells. Using immunohistochemistry, this study evaluated BAFF expression in labial salivary gland biopsies from 14 patients with pSS, 14 normal controls, and two patients with sarcoidosis. Labial salivary gland samples from seven patients with pSS, seven controls, and one patient with sarcoidosis were double-stained using indirect immunofluorescence. RT-PCR analysis was also performed on lip biopsy samples from two patients and two controls. In all 14 pSS specimens, infiltrating inflammatory cells strongly expressed BAFF protein, as did some ductal epithelial cells, but acinar cells were negative. Some B cells were present in the vicinity of the BAFF-positive cells. In the 14 normal labial salivary glands, some ductal cells were moderately positive, but acinar cells were negative. In the labial salivary glands from the two patients with sarcoidosis, infiltrating lymphocytes were not stained. BAFF mRNA expression was confirmed by RT-PCR in salivary glands from pSS patients. Double immunofluorescence revealed T cells and macrophages to be the main cell types expressing BAFF in salivary glands from pSS patients. In conclusion, BAFF may be expressed by T cells at the site of autoimmune damage and could play a role in the pathogenesis of pSS, particularly by triggering the activation of self-antigen-driven autoimmune B cells. Copyright © 2004 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.