Activation of the transcription factor interferon regulatory factor-3 (IRF-3) is an essential event in the innate immune response to viral infection. To understand the contribution of IRF-3 to host defense, we used a systems biology approach to analyze global gene expression dependent on IRF-3. Comparison of expression profiles in cells from IRF-3 knockout animals or wild-type siblings following viral infection revealed three sets of induced genes, those that are strictly dependent on IRF-3, augmented with IRF-3, or not responsive to IRF-3. Products of identified IRF-3 target genes are involved in innate or acquired immunity, or in the regulation of cell cycle, apoptosis and proliferation. These results reveal the global effects of one transcription factor in the immune response and provide information to evaluate the integrated response to viral infection.