Inflammatory responses are associated with significant changes in tissue metabolism. In particular, metabolic shifts during inflammation can result in significant tissue hypoxia, with resultant induction of hypoxia-responsive genes. Given this association, we hypothesized that leukocyte functional responses are influenced by hypoxia. Initial experiments revealed that exposure of the promonocytic cell line U937 to hypoxia resulted in increased adhesion to activated endothelia. Such increases were transcription-dependent and were blocked by antibodies directed against β₂, but not β₁, integrins. Analysis of β₂ integrin mRNA and protein in U937 cells revealed a 5- to 6-fold increase with hypoxia. Extension of this analysis to hypoxic human whole blood revealed prominent induction of β₂ integrin mRNA and protein ex vivo. Furthermore, murine β₂ integrin mRNA was found to be significantly induced during hypoxia in vivo. Subsequent studies identified a binding site for hypoxia-inducible factor 1 (HIF-1) in the CD18 gene. This gene encodes the subunit common to all four known types of β₂ integrin heterodimer. HIF-1 binding was demonstrated in vivo, and mutational analysis of the HIF-1 site within the CD18 promoter resulted in a loss of hypoxia inducibility. Taken together, these results demonstrate that hypoxia induces leukocyte β₂ integrin expression and function by transcriptional mechanisms dependent upon HIF-1.