To assess the clinical and serological outcome of hepatitis delta virus (HDV) infection, 59 patients with acute delta hepatitis were followed for 6-28 months. Forty-two patients had simultaneous HDV and HBV coinfection (anti-HBc IgM-positive, group I) and 17 were HBsAg carriers with HDV superinfection (anti-HBc IgM-negative, group II). Overall, serum HD-Ag and anti-HD IgM were the most sensitive markers for diagnosis of delta infection during the first 2 weeks after onset of symptoms. The clinical presentation was similar in both groups; 4 patients (1 in group I and 3 in group II) (7%) developed fulminant hepatitis, but none of them died. The majority of patients with HBV-HDV coinfection (group I) eventually recovered, whereas all HBsAg carriers with HDV superinfection (group II) developed chronic liver disease. Liver histology in these patients showed chronic active hepatitis and/or cirrhosis in 90%. The hepatic lesion was probably due to persistent HDV infection, as indicated by the presence of intrahepatic HD-Ag and/or persistence of serum anti-HD IgM in 90% of the patients. © 1987 Elsevier Science Publishers B.V. All rights reserved.