The Pathbase mouse pathology ontology provides a description of experimental mouse pathology phenotypes and incorporates 889 concepts including anatomical pathological entities and pathological processes. These are organised in an almost entirely is_a hierarchy of pathological processes or anatomical entities. Questions, comments and suggestions for amendment should be sent to the Co-ordinator, Dr. Paul Schofield, Dept. of Physiology Development and Neuroscience, University of Cambridge (PNS12@cam.ac.uk).The ontology was developed by the Pathbase consortium (http://www.pathbase.net) funded under the European Commission s Fifth Framework programme, Contract number QLRI-CT-1999-00320.