Since the amount of hepatitis C virus (HCV) RNA might be correlated with the degree of severity of hepatitis and response to treatment, quantitation of HCV RNA in serum was established using competitive polymerase chain reaction (PCR). Known amounts of a plasmid containing HCV-cDNA were co-amplified with a standard dilution series of a competitive template which shared the primers' sequences but differed from the wild type cDNA in having a deletion. Accurate quantitation was obtained by comparing the amount of both products. Quantitation of serum HCV RNA was carried out in two patients' serum samples which were also used to infect chimpanzees. The concentration of HCV RNA in these two sera was calculated to be 1 pg/ml (non-infectious at 10(-3) dilution) and 1-10 pg/ml (infectious at 10(-5) dilution). The procedure was subsequently used to analyze serial changes in serum HCV RNA in three patients who were treated with alpha-interferon. During treatment, the levels of alanine aminotransferase showed a significant decrease in all patients and the amount of HCV RNA fell from 1 fg/ml, 1 pg/ml, and greater than 10 pg/ml to 0.1 fg/ml, 100 fg/ml, and 1 pg/ml, respectively. The decrease in the amount of HCV RNA after treatment was related to the initial amount of serum HCV RNA. These results suggest that quantitation of HCV RNA may be useful not only for understanding the course of HCV infection but also for evaluating treatment for HCV infection.