The serologic and viral profiles of 24 patients who presented with acute hepatitis B virus (HBV) infection were studied. Although in rare cases, HBV-DNA was detectable before hepatitis B surface antigen (HBsAg) and e antigen (HBeAg), in the majority the viral proteins appeared first. In acute hepatitis followed by recovery, as IgM anti-HBc (hepatitis B core antigen) titres rose, the level of HBV replication fell and serum transaminases became elevated. In patients progressing to chronic HBV infection, IgM anti-HBc titres rose early, viral replication was initially low but continued to rise as the serum transaminase levels became elevated. 7S IgM anti-HBc, although present in the phase of established chronic HBV infection, was not found in the early phase of the chronic infection. Thus this antibody appears to be a consequence of, rather than a causative factor in, chronic HBV infection.