The prevalence of delta-markers among 71 patients with fulminant B viral hepatitis was found to be 33.8%. The majority of the patients with delta-markers showed serologic evidence of simultaneous acute delta-infection and B viral infection. Only 5 of the 24 patients with serologic markers of acute delta-infection in this fulminant group were presumably infected chronically with hepatitis B virus as shown by the absence of immunoglobulin M antibody to hepatitis B core antigen. A study of serologic markers of acute delta-infection among 118 patients with nonfulminant acute B viral hepatitis, in contrast, revealed only 4.2% incidence. This significant difference in the prevalence of simultaneous acute B and delta viral infections between the fulminant and the nonfulminant acute hepatitis groups indicates a higher morbidity rate associated with simultaneous infection. When the fulminant group was divided into acute B viral infection without delta-markers (subgroup 1), simultaneous acute B and delta-infections (subgroup 2), and chronic asymptomatic B with acute delta-infections (subgroup 3), for comparison of survival data, the mortality rate was not significantly different in the first two groups when the patients were age matched.