The IMGT/HLA database (http://www.ebi.ac.uk/imgt/hla) has provided a centralized repository for the sequences of the alleles named by the WHO Nomenclature Committee for Factors of the HLA System for the past four years. Since its initial release the database has grown and is the primary source of information for the study of sequences of the human major histocompatibilty complex. The initial release of the database contained a limited number of tools. As a result of feedback from our users and developments in HLA we have been able to provide new tools and facilities. The HLA sequences have also been extended to include intron sequences and the 3' and 5' untranslated regions in the alignments and also the inclusion of new genes such as MICA. The IMGT/MHC database (http://www.ebi.ac.uk/imgt/mhc) was released in March 2002 to provide a similar resource for other species. The first release of IMGT/MHC contains the sequences of non-human primates (apes, new and old world monkeys), canines and feline sequences. Further species will be added shortly and the database aims to become the primary source of MHC data for non-human sequences.