Serum cytokines are thought to be involved in autoimmune hepatitis (AIH) pathogenesis via immune dysregulation. B-cell activating factor belonging to the tumor necrosis factor family (BAFF) is a member of the tumor necrosis factor (TNF) superfamily and is known for its role in the survival and maturation of B cells. The aim of the study was to evaluate the serum levels of BAFF in patients with AIH and determine its relation to the clinical features of AIH. We examined serum BAFF levels in 55 patients with AIH, 14 patients with acute hepatitis (AH), 33 patients with chronic hepatitis C, and 33 healthy subjects by enzyme-linked immunosorbent assay. Liver function tests, quantitative immunoglobulin, and antinuclear antibody levels were also assayed in AIH patients. Serum BAFF levels were elevated in AIH patients compared with healthy subjects (AIH: 2.07+/-1.21 pg/ml, control: 0.77+/-0.22 pg/ml). Similarly, serum BAFF levels were significantly higher in AIH patients compared with AH or chronic hepatitis C patients. There was a positive correlation between BAFF and aspartate aminotransferase (r=0.513, p<0.0001), alanine aminotransferase (r=0.435, p<0.0001), total bilirubin (r=0.419, p<0.01), and soluble CD30 (r=0.579, p<0.0001) in AIH patients. However, there was no correlation between BAFF and levels of gammaglobulins or titer of antinuclear antibodies. Corticosteroid treatment resulted in marked reduction in serum BAFF levels in AIH patients. These results suggest that BAFF contributes to liver injury and disease development in AIH patients.