To evaluate the potential pathogenicity to mammals of the recent H5N1 avian influenza A virus, viruses recovered from dead mice infected with A/chicken/Yamaguchi/7/2004 isolated in Japan were examined. All recovered viruses from the brains of dead mice infected with this strain (without any prior adaptation to mice) had substituted the amino acid at position 627 of the PB2 protein from glutamic acid to lysine. Their mouse lethality had increased by approximately 5 x 10(4) times over that of the original virus. Histopathological analysis reinforced the finding that these variants caused more rapid and severe damage to mice than the original virus. This revealed that it might be useful to characterize the recovered virus to assess its potential pathogenicity to mammals.