The receptors for interferon-α/β (IFN-α/β) and IFN-γ activate components of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway, leading to the formation of at least two transcription factor complexes1. STAT1 interacts with STAT2 and p48/IRF-9 to form the transcription factor IFN-stimulated gene factor 3 (ISGF3). STAT1 dimers form γ-activated factor (GAF). ISGF3 is induced mainly by IFN-α/β, and GAF by IFN-γ, although both factors can be activated by both types of IFN. Individuals with mutations in either chain of the IFN-γ receptor (IFN-γR) are susceptible to infection with mycobacteria2-5. A heterozygous STAT1 mutation that impairs GAF but not ISGF3 activation has been found in other individuals with mycobacterial disease6. No individuals with deleterious mutations in the IFN-α/β signaling pathway have been described. We report here two unrelated infants homozygous with respect to mutated STAT1 alleles. Neither IFN-α/β nor IFN-γ activated STAT1-containing transcription factors. Like individuals with IFN-γR deficiency, both infants suffered from mycobacterial disease, but unlike individuals with IFN-γR deficiency, both died of viral disease. Viral multiplication was not inhibited by recombinant IFN-α/β in cell lines from the two individuals. Inherited impairment of the STAT1-dependent response to human IFN-α/β thus results in susceptibility to viral disease.