The NS1 protein of human influenza A viruses binds the 30-kDa subunit of the cleavage and polyadenylation specificity factor (CPSF30), a protein required for 3' end processing of cellular pre-mRNAs, thereby inhibiting production of beta interferon (IFN-β) mRNA. The NS1 proteins of pathogenic 1997 H5N1 viruses contain the CPSF30-binding site but lack the consensus amino acids at positions 103 and 106, F and M, respectively, that are required for the stabilization of CPSF30 binding, resulting in nonoptimal CPSF30 binding in infected cells. Here we have demonstrated that strengthening CPSF30 binding, by changing positions 103 and 106 in the 1997 H5N1 NS1 protein to the consensus amino acids, results in a remarkable 300-fold increase in the lethality of the virus in mice. Unexpectedly, this increase in virulence is not associated with increased lung pathology but rather is characterized by faster systemic spread of the virus, particularly to the brain, where increased replication and severe pathology occur. This increased spread is associated with increased cytokine and chemokine levels in extrapulmonary tissues. We conclude that strengthening CPSF30 binding by the NS1 protein of 1997 H5N1 viruses enhances virulence in mice by increasing the systemic spread of the virus from the lungs, particularly to the brain.