It has been found that almost all haemophiliacs treated with pooled concentrates of clotting factor VIII or IX before 1985/6 have been infected with hepatitis C virus (HCV). In order to clarify the characteristics of HCV infection in Japanese haemophiliacs, we investigated the HCV genotype and HCV-RNA level in 80 patients with haemophilia who had been confirmed to be positive by a second-generation HCV antibody test. HCV-RNA was detected in 60 (75.0%) individuals and various HCV genotypes were found. Although 80% (48/60) of the patients had genotype 1b, the frequency of each genotype was quite different from that in HCV-infected non haemophiliac Japanese. Particularly, multiple HCV genotypes were observed in 27 (46.7%) patients. The mean (+/- SD) level of HCV-RNA was 5.3 x 10(5) +/- 1.1 x 10(6) copies mL(-1). The viral load in patients with genotype 2a was significantly less common than those with genotype 1a (P = 0.0007), genotype 1b (P = 0.0009) and combined genotype 1a/1b (P = 0.0019). In patients co-infected with human immunodeficiency virus (HIV), the HCV-RNA level was significantly higher (P = 0.05) than in those without co-infection. However, there was no significant difference (P = 0.25) in the HCV-RNA level with HCV/HIV co-infection among the 40 patients with group 1 genotypes. We conclude that this biased distribution of HCV genotypes in Japanese haemophiliacs reflects their specific mode of HCV infection. Moreover, these results suggest that super-infection with HIV does not greatly influence the HCV load in patients with no marked immunological deterioration.