The diagnostic and prognostic significance of IgM anti-HBc, studied by a solid phase u-antibody capture radioimmunoassay at a serum dilution of 1:4000, was prospectively evaluated in 73 adult patients with acute hepatitis seropositive for hepatitis B surface antigen (HBsAg). Of the 73 cases, 20 (27.4%) cleared their HBsAg within 6 months, while the remaining 53 (72.6%) did not. HBsAg seroconversion to its antibody occurred in 15 (93.8%) of the 16 patients positive for IgM anti-HBc with S/N ratios above 5.0, as did 5 (26.3%) of the 19 with S/N ratios between 2.1 to 5.0, and none (0%) of the 38 negative for IgM anti-HBc (S/N ratios less than 2.1). Therefore, a S/N ratio of IgM anti-HBc above 5.0 is diagnostic for acute type B hepatitis. However, low S/N ratios (2.1-5.0) of IgM anti-HBc were observed in the early stage of some patients with acute type B hepatitis, and would increase to a level greater than 5.0 when assayed again 1-2 weeks later. It was therefore suggested that repeated testing of anti-HBc IgM is mandatory for accurate diagnosis of acute type B hepatitis in patients whose initial serum specimens showed low S/N ratios of IgM anti-HBc. According to this criterion, only 22 (30.1%) of the 73 patients with acute hepatitis seropositive for HBsAg in Taiwan were true acute type B hepatitis, of whom 2 (9.1%) subsequently became chronic HBsAg carriers, while the remaining 51 (69.9%) were chronic HBsAg carriers with other superimposed forms of acute hepatic injury.