The B-cell activity factor (BAFF) acts as a positive regulator of B-cell function. To gain further insight into the understanding of B-cell hyperactivity in autoimmune diseases, the serum level of BAFF was determined in 43 systemic lupus erythematosus (SLE) patients, 58 primary Sjögren's syndrome (pSS) patients, 28 rheumatoid arthritis (RA) patients, and 68 normal control subjects using an in-house sandwich ELISA. A commercial kit was used to detect soluble CD23 (sCD23) reflecting B-cell activation. In-house assays for the detection of autoantibodies also were used. We found an increased level of BAFF in SLE, pSS, and RA sera compared with normal subjects (respectively, 10.6 +/- 8.5, 15.8 +/- 12.9, 9.7 +/- 1.5 ng/mL vs. 4.6 +/- 2.9 ng/mL, P < .001). sCD23 released on B-cell activation also was found to be elevated in SLE, pSS, and RA compared with normal sera. However, no correlation was found between the circulating BAFF and the level of sCD23. By contrast, we observed that high levels of BAFF were associated with the presence of autoantibodies (anti-double-stranded DNA antibodies in SLE, anti-SSA antibodies in pSS, and rheumatoid factors in RA). Our data suggest that BAFF is influential in driving antibody production rather than activation of the B lymphocytes in autoimmune diseases.