The value of anti-HD IgM as a marker of chronic delta infection was evaluated by correlating its presence in serum with that of HD-Ag in liver cells and with the degree of inflammatory activity. Thirty-six patients with HBsAg-positive chronic hepatitis and anti-HD at high titers were studied. Overall, the liver cells of 26 patients contained HD-Ag and 27 were positive for IgM anti-HD. The correlation between both markers was excellent: 25 cases were positive for both serum anti-HD IgM and intrahepatic HD-Ag and 8 were negative for both markers. There was only 1 HD-Ag-positive patient, who was anti-HD IgM-negative. Two patients were anti-HD IgM-positive and HD-Ag-negative. Histological damage was more severe in anti-HD IgM-positive cases than in those negative for this marker (Knodell's index 13.5 vs 11.9, P < 0.01). We conclude that anti-HD IgM is a good marker of chronic active delta infection. © 1987 Elsevier Science Publishers B.V. All rights reserved.