The IgM class antibody to the hepatitis delta virus (HDV) was determined in different clinical categories of hepatitis B surface antigen carriers infected by the HDV (positive in the test for total antibody to HDV). The IgM antibody was found at high titers in each of 70 patients with inflammatory liver disease and at a low titer in one of six patients with inactive cirrhosis; it was not found in eight carriers with normal liver histology. The IgM antibody persisted in high titer over many years in patients with unremitting or progressive liver disease, but declined or disappeared before the homologous IgG antibody in the patients whose disease improved or resolved. Testing for IgM antibody to HDV distinguishes hepatitis B surface antigen carriers who have underlying inflammatory HDV liver disease from those with past HDV infection and provides prognostic information on the course of chronic HDV hepatitis.