Markers of hepatitis B virus (HBV) and delta agent were prospectively tested in sera of 107 intravenous drug abusers with acute hepatitis positive for hepatitis B surface antigen (HBsAg) associated with delta infection and compared with the findings in addicts with acute classical hepatitis B. On the basis of the presence and titer of IgM antibody to hepatitis B core antigen, 86 of the addicts with delta infection had simultaneously acquired HBV and delta agent, and 21 were chronic carriers of HBsAg experiencing acute delta superinfection. The frequencies of biphasic and severe hepatitis were significantly higher (P less than .05) in delta agent-infected patients than in controls, but the acute clinical and biochemical features of the two varieties of delta disease were not distinguishable. However, in analogy to the clinical outcome of classical hepatitis B, all patients with nonfatal acute HBV/delta coinfection had self-limited illness, whereas 20 of 21 HBsAg carriers superinfected by delta agent developed chronic active hepatitis.