Since publication of the polymerase chain reaction (PCR) technique in 1985 (Saiki et al. Science 1985; 230: 1350-1354), there has been an explosion of reports on its use in medicine and science. We critically review its use both as a diagnostic technique and as a research tool, and show the pathologist how to evaluate PCR data and how to avoid the pitfalls of overinterpretation. We discuss the value of PCR in the characterization of genetic defects, prenatal diagnosis, carrier testing, HLA typing, detecting micro-organisms, identifying activated oncogenes, and in the characterization of leukaemias and lymphomas, and summarize the main applications in biomedical research.