Advances in microarray technology and the sequencing of the human genome are opening up new possibilities for applying genomic information in clinical medicine, using information about structural polymorphisms in DNA and changes in gene expression as measured by mRNA. Gene expression profiling studies in cancer samples have led to the identification of clinical signatures that are already being applied in some centers. In stroke, it may be possible to use peripheral blood as a source of mRNA to study gene expression. After stroke, there is a selective recruitment and migration of white blood cells to the ischemic focus in the brain. This appears to involve all white cell types and is believed to impact significantly on tissue and clinical outcome through the exacerbation of ischemic injury, particularly after reperfusion, on the one hand, and conversely contributing to tissue remodeling and repair days to weeks after stroke. The first results from clinical studies in ischemic stroke suggest that a gene expression signature can be demonstrated from peripheral white blood cells and that this represents at least a partial adaptation to the altered cerebral microenvironment. Further studies are indicated to see whether these methods may lead to new management approaches for stroke.