We evaluated the specificity and clinical relevance of anti-hepatitis C virus antibody positivity in 22 HBsAg-negative patients with autoimmune (anti-nuclear, anti-actin or anti-liver-kidney microsomal antibody positive) chronic active hepatitis. An ELISA anti-HCV test and a recombinant immunoblot assay (RIBA-HCV) were used. Thirteen patients (59%) were anti-HCV positive and five (23%) anti-HCV negative by both ELISA and RIBA-HCV tests. Four patients (18%) were borderline positive by ELISA (OD less than 1.0), and three of them (all with severe disease) were negative by RIBA. Histologic necroinflammation, AST/ALT and gamma-globulins levels were higher and response to prednisolone treatment was better in RIBA anti-HCV-negative than in anti-HCV-positive cases. We confirmed with both RIBA and ELISA tests the high prevalence of anti-HCV already reported by ELISA in anti-nuclear and anti-liver-kidney microsomal antibody positive chronic active hepatitis. False positive for anti-HCV (i.e., a positive ELISA test not confirmed by RIBA) occurred only among patients with severe disease. Since RIBA-negative subjects showed the best response to corticosteroid, they might represent the only subset of cases of 'true' autoimmune chronic active hepatitis.