We estimated HCV RNA in serum of 20 patients with chronic hepatitis C during interferon therapy using a combination assay of reverse transcription and polymerase chain reaction (RT-PCR). RNA was extracted from 200 microliters of serum. The primers were chosen from the NS3 region of prototype HCV RNA sequence. After 40 cycles of PCR, the products were analyzed by Southern hybridization. HCV RNA was detected in 18 of 20 (90%) patients before therapy. In cases with improvement of serum GPT level, HCV RNA became undetectable at 4 weeks of the therapy. However, HCV RNA reappeared within 4 weeks after the therapy in cases with relapse. In the no response group, HCV RNA did not disappear during the therapy. Interferon therapy is beneficial in improvement of viremia of HCV, and the detection of HCV RNA in serum is useful to evaluate the antiviral effect of interferon.