To determine the role of serum soluble interleukin-2 receptor (sIL-2R) in chronic hepatitis B virus (HBV) infection, the level of serum sIL-2R was measured in sera of 105 patients with chronic HBV infection and in 21 healthy controls, using enzyme-linked immunosorbent assay. Serum sIL-2R levels were significantly higher in chronic HBV-infected patients with chronic hepatitis (508+/-310 units/ml) and liver cirrhosis (543+/-283 units/ml) than in healthy controls (331+/-106 units/ml, P < 0.05). Moreover, serum sIL-2R levels were significantly higher in patients with chronic hepatitis or liver cirrhosis than in asymptomatic HBV carriers (341+/-150 units/ml, P < 0.01). There was no difference in serum sIL-2R levels between asymptomatic HBV carriers and healthy controls or between patients with chronic hepatitis and liver cirrhosis. A significant relationship was found between serum sIL-2R and ALT levels (P < 0.05) in patients with chronic HBV infection, although there was no correlation between sIL-2R and HBV DNA levels. Serum sIL-2R levels in most patients decreased to the same level as asymptomatic HBV carriers and healthy controls at 48 weeks after the end of treatment, and serum ALT and HBV DNA levels were decreased to within the normal range at 96 weeks. Thus, serum sIL-2R levels indicate the degree of liver damage among patients with chronic HBV infection. The serum sIL-2R levels one year after interferon administration may be a useful marker of determined at the effectiveness by this treatment.