We are describing five of a group of 72 cases of acute hepatitis B in which HBsAg was not present when the disease was diagnosed, i.e. two to six days after the appearance of icterus. The role of HBV infection was assessed by demonstrating the absence of anti-HBs during the first few weeks and its appearance at the time of recovery, as well as the presence of anti-HBe, high anti-HBc titers and the IgM class of immunoglobulin. The high levels of transaminases observed in early sera returned to normal values within two to six weeks. Anti-HAV antibodies were absent in all serum samples tested. These case reports should remind us that the absence of HBsAg at the onset of acute viral hepatitis does not exclude the possibility of HBV infection. Therefore, if HBsAg and anti-HAV are not present at the beginning of acute viral hepatitis, serum should also be tested for anti-HBc IgM before the diagnosis of non-A, non-B hepatitis is considered.