B-lymphocyte stimulator/B activating factor (BLyS/BAFF) is a tumour necrosis factor-family cytokine that plays a key role in generating and maintaining the mature B-cell pool. BLyS/BAFF expression by macrophages is stimulated by interferon-γ and interleukin-10, and its serum levels are increased in chronic hepatitis C (CHC). The aim of this study was to assess serum levels of BLyS/BAFF in patients with acute hepatitis C (AHC) and correlate them with disease outcome. We studied 28 patients with AHC (14 males, mean age 59.3 ± 15 years), followed for at least 7 months since onset, comparing them with 86 CHC patients and 25 healthy blood donors (HBD). BLyS/BAFF levels were assessed at baseline (within 4 weeks of onset) and during follow-up. BLyS/BAFF median levels were significantly higher in AHC (1485 pg/mL) than in CHC (1058 pg/mL) and in HBD (980 pg/mL) (P < 0.001). BLyS/BAFF levels were higher in AHC patients evolving to chronicity (1980 pg/mL) than in those with a self-limited course (1200 pg/mL), (P = 0.02). By logistic regression analysis, higher BLyS/BAFF levels were independently associated with persistence of HCV infection (OR 29.7; 95% CI: 1.73-508.20). High serum levels of BLyS/BAFF at onset of AHC can predict its evolution to chronic infection.