In genomic diagnosis the ensemble of techniques has been recently expanded by the powerful method of the polymerase chain reaction (PCR). Using pairs of synthetic oligonucleotides for priming of synthesis and a thermoresistant DNA polymerase a millionfold amplification of target DNA sequences from patients provides DNA fragments for following investigations such as electrophoresis. The paper presents some examples of PCR application for diagnosis in cystic fibrosis and Duchenne muscular dystrophy.