Tension wood formed in response to gravitational force is a striking example of the plasticity of angiosperm wood. In this study our goal was to characterize the early changes in gene expression during tension wood formation in Eucalyptus. Using cDNA array technology, transcript profiling of 231 genes preferentially expressed in differentiating Eucalyptus xylem was followed from 6 h to 1 wk of a tension time course of artificially bent Eucalyptus trees. 196 genes were differentially regulated between control and bent trees, some exhibiting distinctive expression patterns related to changes in secondary cell wall structure and composition. For instance, expression of a cellulose synthase gene was well correlated with the appearance of the G-layers. Cluster correlation analysis revealed differential regulation of lignin biosynthetic genes and may also be used to help infer the function of unknown gene products. Eucalyptus wood transcriptome analysis during tension wood formation not only provided new clues into the transcriptional regulatory network of genes preferentially expressed in xylem, but also highlighted candidate genes responsible for the genetic and environmentally induced variation of wood quality traits.