The basis for the effective treatment and cure of a patient is the rapid diagnosis of the disease and its causative agent, which is based on the analysis of the clinical symptoms coupled with laboratory tests. Although rapid advance­ ments have been made in the laboratory diagnosis of virus diseases, the neces­ sary isolation of the causative virus from the clinical specimens is a relatively long procedure. Viruses which integrate into the cellular DNA (such as human immunodeficiency virus, HIV -1, or hepatitis B virus) are difficult to identify by molecular techniques, while viruses which exist in the clinical material in low concentrations are even more formidable to identify. Recently, the application of the polymerase chain reaction (peR) technique developed by K. D. Mullis and detailed in the study by Saiki et al. (1985) led to a revolution in virus diagnosis. The peR technique was rapidly applied to the diagnosis of viruses in clinical material. Volume 1 of Frontiers of Virology provides new information on the advantages of the use of the peR for the diagnosis of many human disease-causing viruses, as well as on some problems with its use.