In recent months newer concepts have evolved in our understanding of infection with the viruses that cause acute viral hepatitis. The natural course of hepatitis A has been described, and reliable diagnostics for its identification are now available. The early development of serologic assays for hepatitis B virus infection resulted in a rapid expansion of our knowledge of the serologic identification of this virus and of the natural course of the agent. Improved serologic tests have shown that infection with the virus is far more common than was appreciated in previous years. Its association with the development of chronic liver disease in up to 10% of infected patients is well documented. Among the most exciting events in our understanding of viral hepatitis has been the development of an assay to detect antibody to hepatitis C virus. This has enabled us to determine that posttransfusion hepatitis is usually due to a single hepatitis C viral agent. Unfortunately, the available antibody assay is associated with a high degree of false positivity and requires the utilization of a secondary test for specificity or a naturalization test to identify true positives. It is clear, however, that a person who has this antibody and who is also positive for a secondary test for specificity is likely to harbor an infectious agent in his or her blood. Hepatitis C is associated with an unusually high degree of chronicity, exceeding 50% in many studies. Second-generation assays have already been developed, and it is likely that we will shortly see a great expansion of our serologic diagnostic capabilities.(ABSTRACT TRUNCATED AT 250 WORDS)