To assess the frequency and significance of immunoglobulin M (IgM) antibody to hepatitis B core antigen (anti-HBc) in corticosteroid-treated severe chronic active hepatitis B, we tested 96 serum samples from 16 patients who were seropositive for hepatitis B surface antigen (HBsAg) (group 1) and 8 HBsAg-negative, anti-HBc-positive patients (group 2) by enzyme-linked immunoassay. Samples obtained in the presence and absence of disease activity before, during, and after long-term corticosteroid therapy (mean duration, 42 +/- 7 months) were evaluated. Seropositivity for IgM antibody was demonstrated in 12 group 1 patients, including 9 tested before corticosteroid therapy; no group 2 patients were seropositive. Seropositivity was more common in serum samples obtained during active than during inactive disease (51% versus 22%; P less than 0.05) and more frequent in serum samples that contained hepatitis B e antigen (46% versus 11%; P less than 0.02) and hepatitis B virus deoxyribonucleic acid (50% versus 24%; P less than 0.05) than in those without these markers. In some patients, seropositivity persisted or recurred intermittently during corticosteroid therapy for up to 57 months. We conclude that seropositivity for IgM antibody can be demonstrated frequently by enzyme-linked immunoassay in corticosteroid-treated patients with severe disease. Seropositivity reflects active virus replication, and it is commonly associated with inflammatory activity. The duration of seropositivity may be protracted during long-term corticosteroid therapy.