The endothelium expresses a large repertoire of genes under apparent transcriptional control of biomechanical forces, many of which are neither cell-type nor flow specific. We set out to identify genes that are uniquely flow responsive in human vascular endothelial cells. Transcriptional profiling using commercial DNA microarrays identified 12 of 18 000 genes that were modulated at least 5-fold after 24 hours of steady laminar flow (25 dyne/ cm2). After a 7-day exposure to unidirectional pulsatile flow (19 ± 12 dyne/cm2), only 3 of 12 remained. elevated at least 5-fold. A custom microarray of ∼300 vas cular cell-related gene fragments was constructed, and expression analysis revealed that many flow-induced genes are also induced by at least one of the following agents: tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), transforming growth factorβ, vascular endothelial growth factor, or thrombin, indicating a more general role in adaptive or stress responses. Most flow-Induced genes were also induced by TNF-α but not IL-1β, suggesting the involvement of reactive oxygen species. A limited panel of genes that are unique for flow-exposed cultures was identified, including lung Krüppellike factor (LKLF/KLF2) and cytochrome P450 1B1 (CYP1B1). In marked contrast, both these genes were substantially repressed by TNF-α. LKLF but not CYP1B1 mRNA was detected exclusively in the vascular endothelium of healthy human aorta by In situ hybridization and appeared to be flow regulated. To date LKLF is the first endothelial transcription factor that is uniquely induced by flow and might therefore be at the molecular basis of the physiological healthy, flow-exposed state of the endothelial cell.