Through intravenous drug abuse, this 20-year-old man contracted chronic active hepatitis due to hepatitis D superinfection of hepatitis B. The correct diagnosis, made after a hiatus of 20 years, awaited advances in the understanding of viral hepatitis, the discovery of the hepatitis D virus, and the development of immunostaining techniques that could be applied to his liver biopsy specimen. The case illustrates how immunoperoxidase staining may be done retrospectively and may be particularly useful in diagnosis of individual cases where serologic testing is not available or possible, as well as in scientific surveys of HBV and HDV infection. Since staining for HBcAg and HDAg can provide clinically useful information not available by other methods, it should be considered as a part of the assessment of the liver biopsy in every patient with HBV infection.