To determine if hepatitis C virus infection influences the behavior of type 1 autoimmune hepatitis and to assess the performance parameters of third-generation immunoassays for viral infection in this disease, 64 patients with different patterns of disease behavior were assessed retrospectively for antibodies to hepatitis C virus by third-generation enzyme-linked immunosorbent assay and recombinant immunoblot assay and for HCV RNA by polymerase chain reaction. Hepatitis C virus RNA was detected in seven patients (11%) and antibodies to hepatitis C virus were found in five (8%). All patients who had an acute onset of illness or who sustained remission after therapy lacked HCV RNA in serum. In contrast, four of 31 patients who relapsed (13%) and three of 17 patients who failed treatment (18%) had HCV RNA in serum. Patients with HCV RNA were indistinguishable from those without HCV RNA; in three patients, infection was recognized only by testing for HCV RNA. Four of seven patients with HCV RNA responded fully to corticosteroids, although each relapsed after drug withdrawal. Smooth muscle antibodies (43% versus 91%, P = 0.006) and concurrent smooth muscle and antinuclear antibodies (0% versus 60%, P = 0.003) occurred less frequently in patients with HCV RNA than in counterparts without HCV RNA. The specificity of the third-generation enzyme immunoassay was 98% and its overall predictability was 94%. Its sensitivity, however, was 57% and false positive results occurred in 20%. Hepatitis C virus infection is an uncommon determinant of disease behavior in type 1 autoimmune hepatitis, but it may be present in relapse or treatment failure.(ABSTRACT TRUNCATED AT 250 WORDS)