The a/b-hydrolase fold is characterized by a b-sheet core of ®ve to eight strands connected by a-helices to form a a/b/a sandwich. In most of the family members the b-strands are parallels, but some show an inversion in the order of the ®rst strands, resulting in antiparallel orientation. The members of the superfamily diverged from a common ancestor into a number of hydrolytic enzymes with a wide range of substrate speci®cities, together with other proteins with no recognized catalytic activity. In the enzymes the catalytic triad residues are presented on loops, of which one, the nucleophile elbow, is the most conserved feature of the fold. Of the other proteins, which all lack from one to all of the catalytic residues, some may simply be `inactive' enzymes while others are known to be involved in surface recognition functions. The ESTHER database (http:// bioweb.ensam.inra.fr/esther) gathers and annotates all the published information related to gene and protein sequences of this superfamily, as well as biochemical, pharmacological and structural data, and connects them so as to provide the bases for studying structure±function relationships within the family. The most recent developments of the database, which include a section on human diseases related to members of the family, are described.