The European Bioinformatics Institute has discussed the possibility of establishing a public repository for DNA microarray-based gene expression data with many of the major laboratories developing and using these technologies in Europe and the USA, and established a consensus that the time is right to develop standards for storing these data. There are many reasons for establishing a public repository of these data. First, by storing data obtained from different experiments under varying conditions, the repository will build up progressively more detailed expression profiles for genes and aid in the study of functional genomics. Second, it will facilitate the cross-validation of data obtained by different technologies, characterise various techniques and establish error rates, benchmarks and gold standards. Third, by making these data available publicly, bioinformatics and analysis of gene expression data will be promoted and pushed forward. Fourth, that the experimental data should be public will be consistent with the policy of most journals with regard to the verifiability of published conclusions. As with biological sequence and molecular structure public repositories, this will not prevent submitters from being the first to exploit and publish their data from gene expression studies. Currently the EBI is establishing a pilot database containing the microarray gene expression data either available publicly or provided to the EBI by laboratories generating these data. We would like to encourage laboratories wishing to discuss submitting their data in such a database to contact us.