The human genome project has attracted a great deal of attention in recent years among the general public as well as the scientific community. Although it is likely to be a number of years before many of the expected benefits of the genomics revolution are realized, the impact of these scientific breakthroughs on diagnostic pathology is likely to become apparent relatively quickly. In particular, gene array technology, which allows gene expression measurements of thousands of genes in parallel, provides a powerful tool for pathologists seeking new markers for diagnosis. Several recent studies demonstrate how the gene array approach can not only recognize markers for known categories of neoplasia but also lead to recognition of different categories not previously appreciated. Although this approach shows great potential, the successful application of gene arrays to diagnostic problems will require thoughtful interpretation, just as immunochemical technologies require careful planning and analysis.