The occurrence of thyroid abnormalities and the appearance of organ- and non-organ-specific autoantibodies during long-term recombinant interferon alpha-2a (IFN-alpha) therapy were studied in 86 and 51 consecutive outpatients with hepatitis C and B virus-related chronic active hepatitis (CAH-HCV and CAH-HBV), respectively. Most patients had longstanding community-acquired hepatitis. At baseline, 9.3% of CAH-HCV and 3.9% of CAH-HBV patients showed clinical and/or biochemical signs of thyroid dysfunction. The remaining patients were euthyroid, although anti-thyroid autoantibodies were found in 33/78 (42.3%) of CAH-HCV and in 5/49 (10.2%) of CAH-HBV patients. During IFN-alpha treatment, increased anti-thyroid autoantibody levels were seen in 40% of CAH-HCV initially negative patients, while they became detectable in no more than 10% of CAH-HBV patients. Interferon-alpha-induced hypo- or hyperthyroidism was recorded in 12 of 35 CAH-HCV patients treated for 12 months (34.3%). Only one CAH-HBV patient developed hyperthyroidism. High titers of anti-nuclear autoantibodies (ANA) were recorded at enrollment in 5/36 (13.8%) of CAH-HCV and in 3/16 (18.7%) of CAH-HBV patients. Only one CAH-HCV patient displayed anti-parietal cell antibodies (PCA). After IFN-alpha treatment, ANA were found in 10/28 (35.7%) and PCA in 2/28 (7.1%) of CAH-HCV patients, while an additional CAH-HBV patient developed PCA, but not ANA. However, no signs of systemic autoimmune disease were recorded.(ABSTRACT TRUNCATED AT 250 WORDS)