The pathogenesis of liver fibrosis has evolved dramatically in recent years. Hepatic stellate cells (HSCs) have been recognized as the main fibrogenic cells in the injured liver, and key fibrogenic cytokines have been identified. We propose the use of DNA microarrays to study changes in gene expression in activated HSCs and in fibrotic livers in order to identify key genes involved in liver fibrosis. For this purpose, RNA can be extracted from both cultured cells and liver tissue. The target RNA is hybridized to probes, which are gene-specific sequences immobilized on chips. The hybridization signal is assessed using a confocal laser scanner. Comparison of hybridization signals and patterns allows the identification of mRNAs that are differentially expressed. Statistical analysis enables classification and clustering of genes according to their up- or downregulation. By using this powerful technique, we have identified genes that are differentially regulated in both HSCs and the fibrotic human liver.