Sera from 39 out of 40 patients with chronic hepatitis C virus (HCV) infection who had been treated for 60 weeks with interferon alfa-2b proved initially HCV RNA positive by reversed transcriptase polymerase chain reaction (PCR). These patients were analysed for genotype and quantitatively for HCV RNA levels prior to treatment by using a competitive PCR method with colorimetric detection of the amplified products. HCV RNA levels were correlated to outcome of treatment, mode of acquisition, histology and HCV genotype. The median pretreatment HCV RNA level in sustained responders (n = 15) with eradication of the viremia and normalization of serum ALT levels lasting 24 weeks post treatment was significantly lower than that in the combined group of non-sustained responders (n = 9) and non-responders (n = 15), 2.52 x 10(5) vs 8.90 x 10(5) genome equivalents per ml serum, p < 0.0125, respectively. 10 out of 17 patients with HCV RNA levels lower than the median level (5.64 x 10(5) genome equivalents per ml serum) had a sustained response to interferon treatment versus only 5/22 with levels equal to or higher than the median level, p = 0.04. No significant pretreatment differences in median HCV RNA levels according to mode of acquisition, genotype, or liver histology prior to treatment were seen. It is concluded that a low pretreatment HCV RNA level seems to be indicative of a sustained response to interferon alfa-2b treatment, whereas a high level seems to be indicative of a non-sustained or non-response. In the individual patient, however, the levels varied widely irrespective of response category.