Rheumatoid arthritis and periodontitis are inflammatory diseases modulated by proinflammatory cytokines [e.g. interleukin (IL-1) 1 and tumour necrosis factor α], which activate local fibroblasts to do the following: (1) proliferate, (2) induce gene expression and (3) produce destructive metalloproteinases. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor (composed of HIF-1α and HIF-1ß/aryl hydrocarbon receptor nuclear transporter) that is modulated by hypoxia. HIF-1 binds to and induces several genes containing an HIF-1 consensus binding site, including vascular endothelial growth factor and several glycolytic enzymes. Through differential screening of a human synovial fibroblast cDNA library, we identified HIF-1α as a clone up-regulated by IL-1. The mRNA for HIF-1α subunit was increased 3-4-fold by Northern blot analysis after cells had been incubated for 3 h in the presence of IL-1. In addition, IL-1 increased the binding of the heterodimer HIF-1 to the HIF consensus sequence. These results suggest that HIF-1 might have a role in inflammation, possibly in attempting to re-establish homoeostasis.