The prevalence of serum markers of primary delta infection was determined in 532 patients with acute benign hepatitis B seen in Italy, and in 111 patients with fulminant hepatitis B seen in Italy, France and England. Patients with fulminant hepatitis had significantly higher prevalence of delta markers (43/111, 39%) than did those with benign hepatitis (101/532, 19%). In 25 of the 43 patients with delta-positive fulminant hepatitis, serum markers indicated a primary hepatitis B infection while in the remaining 18, IgM antibody to hepatitis B core antigen was absent, indicating that hepatitis B preceded superinfection with the delta agent. The increased morbidity of HBsAg hepatitis with delta infection may result from the cumulative simultaneous exposure to hepatitis B virus and delta, or from superinfection of HBsAg carriers with delta.