The significance of anti-HCV IgM testing in the diagnosis and evaluation of treatment of chronic hepatitis C was studied. The serum samples from 82 patients with chronic hepatitis C were tested for ALT, anti-HCV IgG and IgM and HCV RNA. All the 82 cases were positive for anti-HCV IgG and 51 (62.19%) positive for anti-HCV IgM. The emergence of anti-HCV IgM might be of four types: (1) persistent positive type, 24 (29.26%) cases; (2) intermittent positive type, 8 (9.75%) cases; (3) transient positive type, 19 (23.17%) cases; (4) persistent negative type, 31 (37.80%) cases. Usually there was progressive liver damage in type 1 and 2. If the patients were of transient positive type, they would usually recover. There was a significant correlation between the levels of serum anti-HCV IgM and ALT, while no correlation existed between the levels of serum ALT anti-HCV IgG. It is suggested that in patients with chronic hepatitis C, detection of anti-HCV IgM may be useful for estimating the extent of progressive liver damage and guiding the antiviral therapy.