Plasmacytoid dendritic cells (pDCs) are vital to antiviral defense, directing immune responses via secretion of huge concentrations of IFN-α. These cells are critical in protecting the lung against clinically relevant respiratory viruses, particularly influenza (Flu), a virus responsible for substantial worldwide morbidity and mortality. How pDC responses to such viral pathogens are regulated, however, is poorly understood in humans. Using an unbiased approach of gene chip analysis, we discovered that Flu significantly affects metabolism in primary human pDCs. We demonstrate that Flu and RV, another common respiratory virus, induce glycolysis in pDCs and that this metabolic pathway regulates pDC antiviral functions, including IFN-α production and phenotypic maturation. Intranasal vaccination of human volunteers with live influenza virus also increases glycolysis in circulating pDCs, highlighting a previously unrecognized potential role for metabolism in regulating pDC immune responses to viral infections in humans.