OBJECTIVE We investigated the prevalence rate of beta 2-glycoprotein I (beta 2-GPI)-dependent antiphospholipid antibodies (aPL)[anti-cardiolipin antibody (aCL), anti-phosphadidylserine antibody (aPS), and anti-phosphatidic acid antibody (aPA)], antinuclear antibody (ANA), anti-deoxyribonucleic acid antibody (aDNA), anti-thyroglobulin antibody (aTG), and anti-thyroid peroxidase antibody (aTPO) in 56 patients with hepatitis C virus (HCV) infection and correlated the results with inteferon-alpha (IFN) treatment. RESULTS aCL, aPS, and aPA were positive in, respectively, 7/56 (13%), 12/56 (21%), and 13/56 (23%) patients before treatment. aPS and aPA appeared in 6/44 and 9/43 and disappeared in 6/12 and 2/13 patients, respectively, after IFN treatment; the differences were statistically significant. The changes in OD readings were higher in the group of patients who became positive for aPS and aPA than in those who became negative after treatment. The positive rates of aTG and aTPO in the patient group were statistically higher than in the healthy controls, and aTG developed in four patients and disappeared in two. No obvious changes in aTPO were observed; however, the aTPO titer was increased in four previously positive patients. None of the patients positive for any antibodies developed an abnormality associated with these antibodies during an observation period of up to 1 yr. CONCLUSIONS The pathogenesis of production and clinical significance of aPL, ATG/aTPO in this clinical setting are unclear, but immunological disturbances, such as the effects of HCV infection and/or IFN treatment, were considered to be a possibility. Further investigation is needed to clarify whether HCV/aPL- and/or aTG/aTPO-positive patients treated with IFN might develop aPL-associated complications and/or autoimmune disease(es), in the future and to confirm whether IFN treatment justifies the outcome in this clinical setting.