The Human Metabolome Database (HMDB) is a freely available electronic database containing detailed information about small molecule metabolites found in the human body. It is intended to be used for applications in metabolomics, clinical chemistry, biomarker discovery and general education. The database is designed to contain or link three kinds of data: 1) chemical data, 2) clinical data, and 3) molecular biology/biochemistry data. The database currently contains more than 2100 metabolite entries including both water-soluble and lipid soluble metabolites as well as metabolites that would be regarded as either abundant (>1 mM) or relatively rare (<1 nM). Additionally, approximately 5500 protein (and DNA) sequences are linked to these metabolite entries. Each MetaboCard entry contains more than 90 data fields with half of the information being devoted to chemical/clinical data and the other half devoted to enzymatic or biochemical data. Many data fields are hyperlinked to other databases (KEGG, PubChem, MetaCyc, ChEBI, PDB, SwissProt, GenBank) and a variety of structure and pathway viewing applets. The database supports extensive text, sequence, chemical structure and relational query searches.