The assay of specific antibodies to hepatitis C virus (anti-HCV) was introduced in the screening of donated blood in November 1989 in Japan and in January 1991 in Thailand. Anti-HCV-positive rates obtained using commercial second-generation kits on donated blood in both countries are similar. However we found differences in the serum alanine aminotransferase (ALT) values of anti-HCV-positive donors between the two countries: 57.0% of anti-HCV-positive Thai donors showed elevated ALT values, whereas only 23.7% of anti-HCV-positive Japanese donors did. Furthermore, different distributions of HCV genotypes were observed among anti-HCV-positive donors of the two countries. Although type 1b showed the highest prevalence among donors in both countries, type 1a showed the lowest prevalence in Japanese donors and the second-highest prevalence in Thai donors. We also examined the HCV RNA levels using a branched DNA probe assay in serum samples of Thai donors and observed no significant relationships between ALT value and HCV genotype or HCV RNA level, although HCV RNA levels in genotype 1b Japanese donors were higher than that in genotype 2a Japanese donors.