The presence of hepatitis B virus and delta agent markers was investigated in 41 patients referred during the years 1970-1985 with fulminant hepatitis classified as type B or non-A non-B and compared to findings in patients with uncomplicated hepatitis B and chronic hepatitis B infection. 13 patients had no markers of hepatitis B and delta infection and were classified as non-A non-B hepatitis. The remaining 28 patients were all HBsAg and IgM anti-HBc positive and 14 (50%) had evidence of delta infection. In contrast, only 13/71 patients (18%) with acute benign hepatitis B had evidence of delta coinfection (p less than 0.005). This corresponds to an odds ratio of 4.5 for development of fulminant hepatitis among patients with hepatitis B and delta coinfection. In 100 chronic HBsAg carriers 29% were positive for delta markers. 12 of the delta infected patients with fulminant hepatitis were positive for total antibody to the delta antigen, and 2 were delta antigen positive. Three were HBeAg positive/anti-HBe negative. None had hepatitis B virus DNA. Among the 14 patients without delta infection, hepatitis B virus DNA was found in 2/4 HBeAg positive/anti-HBe negative patients and in 1/8 patients negative for both markers. The present data indicate that a high proportion of Danish patients with fulminant hepatitis B have hepatitis B and delta agent coinfection. Further, the findings suggest that hepatitis B and delta coinfection may be associated with an increased risk of development of fulminant hepatitis as compared to that of hepatitis B alone.