AbstractA consecutive group of 169 patients with acute hepatitis found negative for hepatitis B surface antigen (HBsAg) and negative for IgM antibody against hepatitis A (anti‐HAV IgM) was studied for presence of IgM antibody against hepatitis B core antigen (anti‐HBc IgM) by ELISA. Anti‐HBc IgM was found in a total of 34 of 60 patients with detectable total anti‐HBc. One hundred and nine patients had no detectable anti‐HBc IgM and no total anti‐HBc and were thus considered as having acute non‐A, non‐B hepatitis. Among the 34 patients with anti‐HBc IgM in their first serum sample, 23 were anti‐HBs negative and all had high and steadily decreasing ratio unit (RU) values for anti‐HBc IgM (mean RU value 17.1). Twelve of the 23 patients showed seroconversion to anti‐HBs during the follow‐up, indicating an actual hepatitis B virus infection. Eleven of the 34 anti‐HBc IgM positive patients had anti‐HBs in their first serum sample. In this group, the RU values for anti‐HBc IgM were high and steadily declining and the initial values were significantly lower (mean RU value 9.9) (p<0.05) than in the anti‐HBs negative group. Evidence is provided that anti‐HBc IgM in serum from patients with HBsAg negative hepatitis with or without anti‐HBs indicates an actual hepatitis B virus infection.According to generally accepted criteria, the demonstration of anti‐HBc IgM identified 20% of the 169 patients with acute non‐A, non‐B hepatiti