We describe a method for quantifying hepatitis C virus RNA in serum. This competitive assay combines reverse transcription and polymerase chain reaction and is based on coamplification of the target RNA with known amounts of synthetic mutated RNA. We tested serum samples from 104 hepatitis C virus carriers (9 asymptomatic blood donors and 95 patients with type C chronic liver disease) to determine the relationship between the replicative level of hepatitis C virus and various stages of the carrier states. The amount of circulating hepatitis C virus RNA ranged from 104 to 109.5 genomes/ml serum. The titer of hepatitis C virus RNA (logarithmic transformed copy numbers of RNA per milliliter of serum) was lower in asymptomatic blood donors (5.4 ± 2.0) and in patients with chronic persistent hepatitis (7.3 ± 1.1) than in patients with chronic active hepatitis (7.9 ± 0.8), cirrhosis (7.8 ± 0.7) or hepatocellular carcinoma (7.9 ± 0.7). The titer of hepatitis C virus RNA was significantly lower in carriers younger than 40 yr old and correlated positively with the logarithmic transformed serum ALT level. Logistic regression showed that age and titer of hepatitis C virus RNA correlated independently with the stages of liver disease. These results showed that the replicative level of hepatitis C virus is higher in advanced liver disease and that elevation of viral replication may play an important role in liver injury and progression of liver disease. This competitive assay is useful in evaluating the state of viral replication in hepatitis C virus infection. (HEPATOLOGY 1993;17:545–550.)