Anti-hepatitis C virus (HCV) immunoglobulin M antibody titres were measured by an enzyme-linked immunosorbent assay method in 16 patients with non-A, non-B acute hepatitis (NANB AH), 13 with non-A, non-B fulminant hepatitis (NANB FH) and nine with type C chronic hepatitis. Anti-HCV IgM was positive in one of the 16 patients with NANB AH, six of the 13 with NANB FH, and five of the nine with type C chronic hepatitis. Anti-HCV IgG was positive in eight of the 16 patients with NANB AH and eight of the 13 with NANB FH. Either anti-HCV IgM or anti-HCV IgG were positive in 10 of the 13 patients with NANB FH. All of the five anti-HCV IgM positive patients with type C chronic hepatitis were undergoing an exacerbation of the diseases, while all of the anti-HCV IgM negative patients were in a remission stage which had lasted for more than 6 months. The findings of this study suggest that anti-HCV IgM is useful for the early diagnosis of type C FH and may be a useful marker of diseases activity in type C chronic hepatitis.