Seventy-five patients (50 M, 25 F), affected by chronic hepatitis caused by hepatitis C virus (HCV), without clinically overt thyroid disease, underwent r-interferon (IFN)-alpha-2a treatment (3-6 MU, 3 times/week) for 12 months. They were tested for thyroid function and for thyroid autoantibodies before (A), 6 (B) and 12 (C) months after the beginning of treatment and after 6 (D) months off therapy. Antithyroglobulin antibodies (Tg-Ab) and TSH were measured by IRMA, antiperoxidase antibodies (TPO-Ab), free T3 (FT3) and free T4 (FT4) by RIA, thyrotropin receptor antibodies (TR-Ab) by RRA. None of the patients showed TR-Ab positivity throughout the study. The number of the patients with one or both antithyroid antibodies progressively increased during treatment (A 10.7%; B 26.7%; C 45.3%) and decreased when off therapy (D 22.7%) with none of them positive for Tg-Ab alone (TPO-Ab 6.7%; Tg-Ab+TPO-Ab 16%). Tg-Ab increased during rIFN (median: A 29.0; B 35.0; C 73.0 U/ml) but decreased when off therapy (D 29.0 U/ml). Instead, TPO-Ab significantly increased throughout the study (A 1.0; B 3.0; C 6.0; D 7.0 U/ml). However, some patients showed for the first time an appearance of antibodies when off therapy. Five patients showed both antibodies and thyroid dysfunction: 2 at B, 2 at C, and 1 at D. Only 1 developed mild transient hyperthyroidism while the other 4 developed hypothyroidism, persistent however only in 1 case. Our study confirms that rIFN-alpha-2a frequently induces thyroid autoimmunity. TPO-Ab seems more useful than Tg-Ab in monitoring the immunological response.(ABSTRACT TRUNCATED AT 250 WORDS)