The 2004 outbreaks of H5N1 influenza viruses in Vietnam and Thailand were highly lethal to humans and to poultry; therefore, newly emerging avian influenza A viruses pose a continued threat, not only to avian species but also to humans. We studied the pathogenicity of four human and nine avian H5N1/04 influenza viruses in ferrets (an excellent model for influenza studies). All four human isolates were fatal to intranasally inoculated ferrets. The human isolate A/Vietnam/1203/04 (H5N1) was the most pathogenic isolate; the severity of disease was associated with a broad tissue tropism and high virus titers in multiple organs, including the brain. High fever, weight loss, anorexia, extreme lethargy, and diarrhea were observed. Two avian H5N1/04 isolates were as pathogenic as the human viruses, causing lethal systemic infections in ferrets. Seven of nine H5N1/04 viruses isolated from avian species caused mild infections, with virus replication restricted to the upper respiratory tract. All chicken isolates were nonlethal to ferrets. A sequence analysis revealed polybasic amino acids in the hemagglutinin connecting peptides of all H5N1/04 viruses, indicating that multiple molecular differences in other genes are important for a high level of virulence. Interestingly, the human A/Vietnam/1203/04 isolate had a lysine substitution at position 627 of PB2 and had one to eight amino acid changes in all gene products except that of the M1 gene, unlike the A/chicken/Vietnam/C58/04 and A/quail/Vietnam/36/04 viruses. Our results indicate that viruses that are lethal to mammals are circulating among birds in Asia and suggest that pathogenicity in ferrets, and perhaps humans, reflects a complex combination of different residues rather than a single amino acid difference.