We assessed the correlation between the positivity for serum IgM antibody to hepatitis C virus and the activity of liver disease in patients with chronic hepatitis C virus infection. Serum samples were taken from 10 antibody to hepatitis C virus-positive asymptomatic patients with normal serum ALT levels, from 14 untreated patients with clinically and histologically proven chronic hepatitis C and from 26 patients with clinically and histologically proven chronic hepatitis C assigned to receive recombinant interferon alpha-2a (6 million IU three times a week for 6 mo). Each serum specimen was tested for IgM antibody to hepatitis C virus-associated C100-3 antigen by enzyme-linked immunosorbent assay. Patients were observed for at least 12 mo. All 10 patients with normal ALT values tested negative for IgM antibody to hepatitis C virus. In contrast, 33 of 40 (82%) patients with chronic hepatitis C had IgM antibody to hepatitis C virus, and a positive correlation was seen between the ALT level and the level of IgM antibody to hepatitis C virus (r = 0.803, p less than 0.001). During interferon treatment, ALT levels declined into the normal range in 18 of 26 treated patients (69%) and remained normal after stopping treatment in 8 patients (31%). In untreated patients, in treated patients who did not respond to interferon treatment and in responder patients who relapsed, no significant changes in IgM antibody to hepatitis C virus levels were seen during the study period. In contrast, IgM antibody to hepatitis C virus became undetectable by the end of interferon treatment in seven of eight patients with a sustained response.(ABSTRACT TRUNCATED AT 250 WORDS)