AbstractSixty‐three intravenous drug addicts with HBsAg positive hepatitis were studied to evaluate the diagnostic usefulness of hepatitis delta virus (HDV) markers for diagnosis of acute HDV infection. Patients were tested for HBsAg, anti‐HBc‐IgM, and anti‐HD‐IgM by radioimmunoassay (RIA), and for hepatitis delta antigen (HD‐Ag) by a commercial enzyme‐linked immunoassay (ELISA). At least two serum samples at a mean interval of 4 wk were examined from each patient. HDV markers were found in 41 cases. In the first serum sample (obtained within 1–5 wk after onset of illness) HD‐Ag was found in 32 cases and was the only HDV marker in 22; in the remaining 10 cases, HD‐Ag was found along with total anti‐HD, and in 6 of them anti‐HD‐IgM was also detected. Five additional patients were only positive for total anti‐HD, and anti‐HD of the IgM class was the only marker in one patient. HD‐Ag was found more often in the patients studied during the first 2 wk of illness.In the second serum sample, HD‐Ag was never the only marker detected, seven patients were still positive, and in all of them anti‐HD was also present. Thirty patients were only positive for anti‐HD. Seroconversion from HD‐Ag to anti‐HD occurred in 20 of 22 (91 %) patients.The results suggest that HD‐Ag determination by ELISA in the initial serum sample, during the first 2 wk of illness, may be the most sensitive test for the diagnosis of acute delta infection, and that seroconversion to anti‐HD u