In 18 patients with clinical evidence of acute hepatitis, positive for HBsAg, and evolution to chronic infection, we detected the presence of antibodies to the delta agent (anti-HDV), in relation to the immunoglobulin serum level. The IgG were significantly higher during the beginning of the acute phase in the 6 positive cases for anti-HDV than in the patients who were anti-HDV negative. In the group positive for anti-HDV, the IgG remained high also during the following months, with a more severe clinical evolution. Among patients positive for anti-HDV there were no significant differences between the initial level of IgG and the presence or not of IgM anti-HBc during the acute phase of the disease (respectively patients with co-infection or super-infection). These results show that the presence of high levels of IgG during the acute phase of HBsAg positive hepatitis suggests the co-existence of delta infection.