The authors describe the development and the application of polymerase chain reaction (PCR) in the detection of hepatitis C virus. Based on the sequence data available in the literature, a conservative motif at the 5' end of the virus genome has been selected for amplification. Using the flanking synthetic oligonucleotides the specificity of the highly sensitive RNA-PCR assay was proved by serologically positive and negative serum samples. HCV-RNA-PCR studies were applied to start interferon treatment in patients from National Institute of Rheumatology and Physiotherapy (Budapest) with anti-HCV positive serum samples and this method was used to evaluate the efficacy of interferon therapy, as well.